If I really had to help you and to make any suggestions, I would have to know you personally, your spouse, your room, your equipment, your musical taste, how loud you play, etc. And even then you might think my suggestion wasn't exactly what you had in mind. 
And please remember: What I hear is not what you will hear - and describing sound just doesn't make much sense.

My suggestions below are only pointing to my own constructions, but these are only a very few examples found on the web. There may be a lot of other constructions out there that may suit your needs and money better.

To summarise:  Please  do not ask what to build, which speaker sounds the best and what is the difference in sound between two speakers.
Thanks! Much appreciated!

Always consider size, cost, complexity and your current skills in loudspeaker building before engaging.

Size (matters)
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This may seem an odd place to start, but nevertheless: You must have an idea about what size speaker you're aiming at. What is the size of your room and do you have a wife who thinks loudspeakers are ugly and should be stuck away on shelves in upper corners of your living room? You need to address these issues first. No point in dreaming about a 100 liter floor-standers if it means divorce.
What I have experienced from mails coming in is that people may have chosen a (small) low-efficiency speaker and "compensated" by buying a powerful amp, and they find out the speaker really can't play that loud all the same...
Well, it doesn't work that way. We must look at the size of the main driver, and if we're dealing with 55-75 square centimeters of membrane area (4" drivers) it simply cannot move a lot of air. You may have heard a speaker with a 10-12" bass driver and we're talking 300-500 cm^2 membrane area. Up to ten times more radiating area compared to a small mini-mini!
Even a 6" driver, typically making some 135-145 cm^2, won't move a lot of air in a vented or closed enclosure. A high-efficiency 6" in a huge rear-loaded horn may be another story, but few people build these things.
Generally people have too high expectations from their diy projects and buying high-quality small drivers won't compensate for the lack of radiating area. So, size matters!

Really small, ~4 liter:
If it must be really, really small you may take a look at Eekels' minis or similar designs. A 4 liter "bookshelf", which, by the way, do better on a pair of solid stands. This is probably as small as it gets - if size is a no-no and it still has to be decent hifi. You may add a small 1 cu. f./30 L sub somewhere in the corner to get some deep bass if need be, but there are limits to how loud a 4" driver will go if you also think of throwing a Saturday night party. Usually very small speakers also have very low efficiency, so 30-40 watts may be minimum and never expect 4" drivers to throw a huge soundstage in a 40 m^2 living room. They can't.

Small, ~7-10 liter:
If you have room for a little more, the W15/OWI may be an option. 7 liter net volume and we get more bass from the 5" driver compared to the 4" driver. Still you won't get it all from this speaker unless you invest in - or build - a pair of solid stands and get it out on the floor. Efficiency is still very low and a decent amp is required. No rock parties either from this speaker.
If 10 liter net volume is OK, there are more options. The SEAS 5INCH projects describe a number of speakers in this category where the CA15RLY provides a lot of good sound for the money.
If you want the best bass possible from a 10 litre speaker I suggest taking a look at the Ellam or Ellam-XT. So, why more bass from this speaker compared to the SEAS-5INCH speakers? All the SEAS 5-inch speakers have a 75 cm^2 membrane area and have higher efficiency (except for the W15CY001) compared to the ScanSpeak's 95 cm^2 membrane area. The price to pay for more bass from this design is reduced efficiency. Nothing comes for free when we're dealing with loudspeakers. Remember, when we reduce efficiency by 3 dB, you need twice the amount of power to render the same sound level - and don't be too sure your drivers like it at all.

Medium size:
So, what's a medium size speaker? Let's say 10-20 litre and either a stand-mount or a small floor-stander. I have a serious lack of this size standmounts in my portfolio. Why? I never could figure out why people would buy a 20 litre stand-mount + a pair of heavy stands - and occupy the same floor space as a pair of true floor-standers would do. It's gotta be something visually. Apparently we "see" an e.g. 15 litre stand-mount as a smaller speaker compared to 25 litre true floor-stander despite occupying the same space in our living room. With the floor-stander we may have 10-15 litre more cabinet volume and for the same money we can significantly increased sensitivity and bass extension, both things so much needed. Stand-mounts apparently have higher WAF compared to a similar sized floor-standers. Dealing with commercial speakers, a given stand-mount may have just the sound you like - and that's another story. No objections here. If you find the one-and-only among the huge number of speakers available in this category, then go for it. The only stand-mount I have in this range is the AT-SW and ScanSpeak Illuminator Monitor, which is a serious investment, I admit, but I don't think you'll be disappointed.
The ScanSpeak CSM range provide a more affordable range on constructions and size can be varied from 13 liter to ~22 liter. And the STUDIO 101 is worth a look in this range. Standmount or bookshelf?

20-40 liter bookshelf/standmount/floorstander:
Usually we're dealing with a 6-6½-7-8 inch basic driver here and we may even try a 2½-way in this category depending on bass driver parametres. Quite a range here: Illuminator Monitor (floor-stander version), Illumina 66, SP95, SP38, Ekta, TJL2W, TJL3W, SEAS CNO, SEAS CNO25, SP98-09, etc. 
The ScanSpeak drivers will deliver deeper bass compared to the 18W- SEAS drivers. The SS 18W/8531 driver is bigger and seems build for bass, where the SEAS drivers may deliver better midrange. The SEAS CNO is my favourite despite not delivering the deep bass of the 18W ScanSpeak drivers, but combined with the excellent SEAS T29 tweeters I find it more lively and the lack of colouration is almost as good as the magnesium drivers. A 2½-way SEAS 18WNX001 has been finished: SEAS CNO25.
In the SEAS low-cost section we find the 3-Way Classic and Poor Man's Strad, both highly popular according to responses. Both speakers deliver some 90 dB sensitivity and an easy load on valve amps.
Latest addition is the NOMEX 164 2½-way speaker from Peerless drivers. I can't recommend this speaker enough if you're looking for a cost-efficient speaker that delivers a midrange that compares to speakers 2-3 times its price.
And take a look at the new (2010) QUATTRO too if you have a valve amp that loves high impedances - or just want the magnificent midrange quality of the JA8008 driver. My new favourite middle-sized two-way.

40-80 liter:
Quite a range here too: Acapella SEas, Acapella NEXT, Ekta Grande, Zahra, Cyclop, Jazzman, Mark21 and Jensen, etc. We're entering a range of speakers that will deliver a significant soundstage. The EktaG, Jazzman and Jensen may deliver the deepest bass of them all followed by the ATR25 (unfortunately discontinued). The Acapella NEXT and ATR25 are my favourites here, not needing a massive power-amp like the Ekta Grande. The sensitivity of the Ekta Grande is around 90 dB/2.8 volts but like most SS constructions, it takes a powerful workhorse to deliver its best. The Acapella NEXT, Zahra and ATR25 are all speakers that can deliver the best from only 50 watts. The constructions listed here will set you back some 1,500-3,000 US $ and don't be stingy on the quality of crossover components. Use good baked coils and super caps where needed, in particular for mid and tweeter.
BTW: I have had reports back on the Ekta Grande saying it's simply too bass-heavy for their rooms. So don't put the EG into a smallish 12-18 square meter room. It can (and should) handle a lot more space.
I had a mail recently asking why there were no reports back on the Jensen speaker. I consider the Jensen speaker the best ScanSpeak construction on these pages and I'm pretty sure why it doesn't attract much attention. First of all the MDF appearance. Unfortunately I never had any photos of the finished speakers from the builder. Maybe they're not finished yet. Next the overall layout may not appeal to diy'ers not being a skinny runt like the Ekta Grande. So, try visualising the Jensen dressed in gorgeous hardwood veneers, piano black or whatever. It's a darn good speaker.
Latest addition to this range is the PRELUDE. Only thing to say about this speaker is this: Make sure you have good amps and source players.

100-200 liter, medium efficiency:
The Jenzen speakers fall into this range (174 litre brutto) and for the time being Jenzen SEAS ER is the economy candidate, but you get an awful lot of speaker for the money. Jenzen NEXT is the ER's big brother and comes at twice the price, but delivers the goodies. 
The Jenzen Accu was a corner stone in the development of the Jenzen speakers, delivering a level of transparency I hadn't had in my living room before.
If you know the ScanSpeak drivers, you may take a look at Jenzen Illuminator.
Icing of the cake is the Jenzen D and Jenzen ATS, my current reference speakers.

80-160 liter high efficiency:
I've only made the TQWT and DTQWT in this volume range and they are in the high-efficiency category of speakers (read below). I've never built a conventional "low-efficiency" speaker this size - and probably never will. Not that it wouldn't be great fun, but the trouble of getting rid of it again is enough for not engaging. I have never made any profit from making finished speakers for sale. A few times I get a price equal to the cost of the components, but all the working hours I have to let go for free. The number of good second hand speakers here in Denmark is humongous and often diy speakers are sold at ridiculously low prices.
I have a lot of requests from people who want to discuss their dream speaker, double 10" bass drivers, etc., and I've promised to make the crossover, but when they start realising the amount of work going into cabinets, overall costs, etc., I usually don't hear any more. Please think it through before writing.

200+ liters:
Take a look at this one: DTQWT-12

Open baffles:
If you have the room and the patience for proper set-up of one of these monster panels, take a look at OBL-11, OB9 and OB7. I dare guarantee the OBL-11 will deliver a live feeling that none of the other speakers mentioned here will ever do. WAF is low, so take care!

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Cost
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Next: Take a deep look into your pockets and decide what amount of money you want to spend on your speaker adventure. Be prepared it will cost more than you think when you scan driver specs and think: Well, a pair of drivers - can't be that serious. You need at lot of components before the project is completed and you may even have to buy additional tools to finish your cabs.
So, is it going to be 500, 1,000, 2,000 or 4,000 US $?? For 2000 US $ you can build a pair of speakers that will compete with some serious high-end commercial speakers at 5-10 times the building cost - that is - if you choose the right combination of drivers and have someone design a proper crossover.
Sometimes commercial speakers are priced deliberately high, because we may think that a cheaper product may not be as good as an expensive one.
Bear in mind also that diy speakers will never have the re-sale value of some well-known commercial speakers. I've seen used ProAc Response 2.5 speakers sold for horrific prices, alone due to the ProAc badge.
Speakers are also a show-off thing, something people may want to own to impress friends - like cars or whatever paraphernalia we buy to create attention. B&O score high here in Denmark for show-off. People may think you're filthy rich (and well, you have to be well off to buy serious B&O stuff!) if you own a whole bunch of B&O gear, where "hardcore" hifi gear like huge horn, fancy turntables and glowing valves may only produce sympathy for the poor spouse having to live with these ugly monsters.

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Bass extension
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People have very different expectations when it comes to overall bass performance. We may all be acquainted with earthquake subs at hifi shows and would like our home speakers to shake our guts as well, but we won't experience anything alike from a modest 22 liter/6" woofer floorstander. I can't be done.
To clarify your requirements on bass extension, visit a hifi store and listen to a range of speakers based on 4", 5", 6", 2 x 6" and bigger bass drivers if available and try to imagine how this will perform in your own listening environment.
You may think that in order to produce decent bass, you need speakers with a linear response down to 30 Hz. Now, 30 Hz would be nice - if the room allows - but I wish it was possible to demonstrate to all first-time diy'ers what it sounds like when we compare a 6" bass driver with -3dB @ 30 Hz to a 12" bass driver with -3 dB @ "only" 50 Hz. Most people would probably agree that bass can mean a lot of things.

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Efficiency
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Efficiency as such is more of academic interest. Efficiency is usually expressed as dB/1 meter/1 watt. As speakers' impedance may vary a lot, sensitivity is what we have to look for, that is: dB/2.8 volts. For valve amps the impedance profile is of particular interest as most valve amps do not like impedances below 4 ohms, where well-designed solid state amps sometimes deliver twice to nominal power in 4 ohms compared to 8 ohms.

Efficiency: My favourite subject, but I won't get into details here, but refer to my pages on high-efficiency speakers listed here:
High Efficiency Speakers intro, discussion

High efficiency speaker constructions:
DTQWT
DTQWT-12
TQWT
OBL-11

Obviously you have to pick a speaker project providing a system sensitivity that your amplifier can cope with. If you have a fine little 10 wpc class A amplifier, a 2.5 clone may not be the best choice, although I've seen exactly this set-up, but this person never did play very loud.
I see a lot of people building 8 wpc 300B amplifiers, but apparently they often run these amps on 85-88 dB speakers! I don't say you can't enjoy the midrange of a 300B amp on these speakers, but they may never reveal all the qualities of your amp.

Most modern speakers have very low efficiency compared to the speakers of the Fifties and Sixties. Low efficiency drivers came around due to the availability of high-wattage solid state amplifiers and the need for small speakers providing decent bass extension, e.g. the "acoustic suspension" (closed box) speakers from Acoustic Research. To do this we need a membrane with high mass and a magnet with low strength and to some people this means "modern hifi" = dull and unexciting reproduction. To others it doesn't matter - and taste cannot be argued.

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