3.5e, but it's the best of a bad bunch.
There's an awful lot that I like about 2nd Edition, but having seen the consistent framework of 3e, I know that I could never go back - there would always be something that I would fell I had to change for my sanity... and then there would always be one more thing.
(It's been a long time since I played BEC(MI) D&D, and while there was a lot I like about it, it has the same issue as 2nd Ed. I haven't played 1st Ed or OD&D to any significant extent, but am reasonably sure that the same applies.)
There's a lot that I like about 4e, but there are also a number of things that I really hate. And some of those things are pretty damn fundamental to the game, so they're not something I feel I can fix with house rules.
So that leaves 3e. And, on balance, I feel that 3.5e was an improvement on 3e. (Actually, Pathfinder is, on balance, an improvement again, but it's not "better enough" to compel me to switch.)
But 3.5e is so damn complex! Our campaign has just hit 8th level, and the range of options the group Wizard, Druid and Artificer have is just absurd. Especially once they start digging in to polymorph magics, and pulling all sorts of wild and wonderful things out the hat.
And that's after we cut a huge swath of complexity from the game by disallowing Prestige Classes and banning everything but a small handful of books!
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Actually, what I really want is, essentially, "D&D Saga Edition" - something with the same fundamental underpinnings as 3e, but with much of the complexity abstracted out. Of course, even "Star Wars Saga Edition" is not without significant flaws, and even it gains some very nasty complexity once you get into all the supplements. But it's the closest thing I've seen to where I would really like to be.