Depends on what you like! They're quite different than one another. The most popular right now is the one that's not named D&D but is Pathfinder. (Which is really the outgrowth of D&D 3rd Edition.)

Kirk or Picard?
Fender or Gibson?
Dick York or Dick Sargent?

Yeah, that just a wee bit subjective. Although it probably makes the least sense, I like the AD&D rules the best. Then again, I don't mind the 4.0 rules either. But probably the most popular is the 3.5 rules. So, as mentioned, Pathfinder, I guess.

I thought today was flying pigs day...

Roughly, in descending order of playability:

3.5ed/3rd ed AD&D - 3rd ed was very modular and felt more balanced than previous editions. There were massive amounts of supplements for it thanks to the Open Gaming License, and the 3.5 edition took the best of all the publshed optional rules and integrated them into the 'core' gameplay experience.

2nd Edition AD&D - A more unified version of AD&D. Brought together some of the de-facto rules from the various sourcebooks and managed to streamline combat a great deal with the introduction of the THAC0 system.

Expert D&D - Personal preference only. More involved than Basic, not as silly as Master.

1st Edition AD&D - kind of raw straight out of the box, as it were. Needs the Unearthed Arcana, Wilderness Survival Guide and Dungeoneers Survival Guide to provide enough rules to make it decently playable for a modern gamer.

Master D&D - too twiddly, but still better than....

Basic D&D - These days, this is little more than a ruleset for quick skirmish boardgame. Nostlagia value only, really.

Kicking an empty tin can down an alley - maybe not everyone's idea of fun, but certainly better than:-

A poke in the eye with a sharp stick - We're definately down at the ass-end of 'fun' here. Still, even this is preferable to:-

4th Edition D&D - Oh Hasbro, I can still feel your steel-capped boots on my boy-parts. And not in a good way. Don't ever go here.

Hmm Pathfinder seems nice. I will also get Planescape and Vampire Masquarade books.

Joel or Mike?
is Spike dead or not?

It depends on what you want.

4th edition has less content, but it's newer and being expanded. The Forgotten Realms setting has a cataclysm, which I'm not a fan of (but you can choose to do whatever story you want). 4th edition feels more "gamey" to me. You get action points you can spend here or there. Everyone rolls the same types of damage and hits (put simply and almost accurately: a sorcerer's magical lance rolls the same as a fighter's longsword). Skills are easy to roll.

D&D 3.5 has more varied rolls. To me, it feels a little more "life-like." Its rules are simpler than AD&D (which is the 2nd version of D&D rules) because they made the math make more sense than their original rules (essentially, they took some negative numbers and made them positive, making the math just a step easier). 3.5 has more content and I'd imagine a larger player base. It looks like WotC is reprinting these, so you can get them new again. However, I like 3.5 also b/c if you have a good used book store around, you can often get great deals on lots of books.

AD&D and D&D1.0 are rule sets that I'd stay away from personally. There's nothing wrong with them, but it's getting harder to find material.

D&D rules were used in really old computer games. None that I played really fully adapted them, but I'm sure there were some. I didn't play much back in those days.. AD&D are the rules used with Baldur's Gate 1&2 and Icewind Dale 1. D&D 3.0 is used by Neverwinter Nights. NWN2 uses 3.5, as does Temple of Elemental Evil. The big changes are that a few spells and a few classes get a boost or a drop in power to make them more desirable or less desirable to use (example: some spells had effects lasting for 24 hours. You'd never not take the spell b/c it simply had too much power. Making it last for a few minutes instead of a few hours makes sure that the spell is useful, but it doesn't wreck the other good spells' chance of getting used).

4.0 hasn't really been used in a game that I've seen. A game on the XBOX I played had adapted some of it, but not really. I'm sure it's debatable, but I think that it would be harder to make 4.0 into a good and fun action-oriented video game without removing or adding a lot to the ruleset b/c it heavily relies on the game being turn-based, but it is good and fun to roleplay around a table with.

My overall recommendation: 3.5.

I bought 4 and then sold it. Our current games we play are all 3.5. It's enough to have fun with and it keeps it a little bit cheaper.

You can take my Bard from my cold, dead hands.

That was hilarious. And quite accurate.
Yes. I forgot to mention that. 4.0 removed some really well-liked classes. They added some races that people like, but you could have done that using the monster manual in 3.5.

Bard is probably my favorite class (at least in 3.5 edition) and the class is available in 4th edition as well.

4th Edition was a very different game from the flexible and multi-faceted format that was 3.5ed. It practically regressed to Basic D&D in terms of the emphasis it placed on combat over any other kind of playstyle - in fact, with it's introduction of fixed 'roles', the removal of non-combat spells and abilities, and the reduction of all class abilities to near-identical spell-like abilities, it read like the pen'n'paper version of an MMORPG.

And, really, no-one wants to get me started on players being able to decide what treasure they get from an encounter and actually being able to tick off the number of encounters they have until they get it....

It might have made a very nice board game. It was not, however, the next evolution of AD&D that we'd been promised.

But hey, I like Rolemaster, so that probably tells you everything you need to know about my own gaming habits :)