3.5, definitely.
The earlier editions were fun, and I enjoyed my time playing 1e and 2e, and 3e, but 3.5 feels like the evolutionary peak of D&D. It combines a fun, stable, decently balanced engine with all the heritage and style and feel of decades of D&D and gaming-style assumptions that fit with the way I like to game. I couldn't go back to 1e or 2e for anything other than a brief nostalgic flashback without thinking "we could do so much better now" (and I'd probably even think it in the flashback, but be able to ignore it for a one-shot or something like that). 3.5 is not perfect, but it's a lot closer than any other edition before or since.
4e? No way. Compared to 3e it's like a Bizarro-clone that looks like D&D but gets everything wrong, or some weird evolutionary offshoot that looks like it also descended from the same distant ancestors but is only barely recognizable now. If you like it, go have fun with it, but it's not The One as far as I'm concerned.
Non D&D/Non-D20? Naah. I've had fun with some of those systems (New WoD, d6 Star Wars, and LUG Trek come to mind), and some of them are pretty dang good, but if there can be only one, then I'll go with 3.5.