This is a seriously tough one.
My game playing has gone in a fairly predictable cycle through the years.
I played Basic D&D and then moved to AD&D (1E). As time went on I would start playing other rpgs that were not D&D. After a number of years I'd want to play D&D again so I picked up the latest edition, which was 2E. Did that for a number of years until I started playing other non-D&D games...and then I picked up 3E when I wanted a D&D fix. Played 3E...then other games...then 4E...and right now I'm playing other games again...and will likely try 5E a couple of years after it comes out.
The thing is, I do not go back to play earlier editions. Every edition has its good points and its so very bad points. I have never felt the need to play D&D because it was D&D. I am not some sort of D&D loyalist. I play it because I think it will be fun...and it has been...every edition.
However, it would be a cop-out if I didn't answer the question so...
2E is my favorite edition.
I liked the fact that character options were a bit more vibrant than 1E, but not as complex as 3E or 4E. It also had some of the best settings ever with Dark Sun, Birthright and Al Qadim. I like all the editions, but if you're going to make me choose, 2E would be it.