This one is a tough call. I'll always love original Third Edition D&D, as it's the edition I started with, but I think my favorite edition is probably 3.5, simply for the amazing wealth of options and different splat books published for that edition.
I don't play much 3.5 much anymore, mostly because it gets fairly complex at higher levels, but I still use a lot of the books for inspiration. The Expanded Psionics Handbook is by far my favorite D&D book, to the point where I consider it to be the fourth core book for 3.5. I love the general aesthetic of the 3.5 books in terms of the artwork, especially the Expanded Psionics Handbook and the first Monster Manual. My only problem with 3.5 (aside from the extreme balance issues between casters and non-casters, which is another post altogether) is that there weren't any official Dark Sun supplements published for it.