Excellent question! And no potential for controversy at all!
In general, I think the best edition of any game is the edition you’re playing and having fun with at any given time. Original AD&D was the D&D I started with, and it was exactly what I wanted it to be while I was playing it. I skipped 2nd Edition AD&D (for reasons having nothing to do with its quality; I think it’s a great revision in its own right), and when I picked up 3rd Edition, it was exactly what I wanted it to be while I was playing it. I worked on a lot of 4th Edition D&D and loved its focus and mechanics. I’ve worked in some way or another on pretty much every official release so far for D&D 5th Edition, and I can attest that it’s as great a game as everyone else is saying it is.
But stacking all the editions up against each other, my favorite would be D&D 3rd Edition. Because 3e was a game that I thought captured the feel of the best AD&D rules in ways that made narrative sense, and I’m a big fan of things making narrative sense. AD&D said, “Your magic-user can’t wear armor and your thief can’t use a shield; that’s just the way it is, suck it up.” 3rd Edition said, “Your wizard can wear full plate and your thief can use a heavy steel shield if you really want them to; but you really don’t want them to, because the following penalties will weigh you down.”
I personally loved that approach, just as I loved the way the 3rd Edition mechanics extended so naturally from AD&D — even when the mechanics were turned upside down, as with the conversion from combat tables and THAC0 to ascending AC. I loved that 3rd Edition saving throws were technically very different than in AD&D but that they felt the same. I loved that the magic items of 3e were the same as those in AD&D, except now there were properly codified rules for how they worked and how they were made. I loved that even as 3e took the game along very different paths than AD&D trod (and I’m the first to admit that some of those directions were ultimately more complex than they needed to be), the gaming journey felt the same as the journey I’d taken all those years ago.
(I also loved the Open Gaming License, with a passion that knows no bounds. After a long hiatus from gaming, it was actually the OGL that got me interested in D&D again — and specifically, the sense that came with the OGL of what sorts of creative possibilities existed within such a license. That sense of open-ended creativity jump-started my interest in the new edition, and ultimately led to me working on it. That’s kind of a side issue, though.)