It's very difficult to decide for me, because I like things about all the editions.
I do like the rules for 3e in theory, but I can't quite wrap my head around them in practice. My games have slowed down considerably since I started running 3e, and my prep time is probably tripled. It's nice to have rules for just about anything I want to design, but it's a two-edged sword.
2e is the edition I have played the most, so it gets the nostalgia points. I wasn't too fond of the glut of settings released, though - much of the fluff never grabbed me. Still, the games I ran under 2e are still my best, and the rules came to me more intuitively than they have with 3e.
1e I haven't ever played, but I've read most of the adventures and the rulebooks, and of all editions this is the one with the best feel. Everything that is cool about D&D today is drawn from this era and OD&D.
The red box Basic Set was my first experience with the game in any fashion, and that fact that I was able to teach myself the game as a 9-year-old just from reading these books speaks wonders for its ease of play - I don't think that would be possible with the game in its current form.
I'm seriously considering going through every edition and cobbling together a version of D&D that includes all of my favourite stuff from each one. The one thing stopping me is that by the time I finish, 4e will be out...