Based on consistent and balanced rules, I'd give 3E the top slot -- no question. However, rules aren't everything, IMO. Based purely on "had the most fun playing 'em," I'd rank them like this:
D&D (classic)
AD&D (1E)
D&D (3E)
AD&D (2E)
(Just as an aside, my "most fun" rankings aren't pure nostalgia, as I played classic D&D just last week. Haven't played AD&D for a while, but I run a C&C game using the C&C PH and the 1st edition DMG, MM, MMII, and FF -- C&C is very similar to AD&D.)
I can break it down a little further for the grognards:
D&D (BECMI/RC - Mentzer)
AD&D (1E)
D&D (B/X - Moldvay/Cook)
D&D (Holmes Basic)
D&D (3.0)
D&D (3.5)
AD&D (2E)
As you can see, I never liked 2E very much (and didn't really play it much, either). I can't put my finger on why, exactly. I remember being really let-down when I finally read the books. The blue interior art sucked. The DMG was a pale shadow of its former self. The writing had lost its magic. Etc.
Here's my "spent the most hours with" list (similar to my favorite list, but a few differences):
AD&D (1E)
D&D (BECMI/RC - Mentzer)
D&D (3.0)
D&D (B/X - Moldvay/Cook)
D&D (Holmes Basic)
AD&D (2E)
D&D (3.5)
Edit - I know I left out the original 3 book edition. That's because I've never really played it or owned it, although I've looked through the books.