Simple question on the surface, right?
It's a bit harder when you get down to the brass tacks, as the answer may be elusive, rational, emotional, unknown.
For me, Swords & Wizardry Complete is my OSR clone of choice. As I've said elsewhere, and on this very blog, it plays very close to AD&D 1e as we played it, not how the AD&D 1e rules were written.
Now, with that being said, Labyrinth Lord was the first OSR clone I considered running. It just didn't hit the sweet note for me that S&W does. Maybe it's because it draws inspiration from the B/X releases in visual presentation and my entry
See, even for me the answer isn't fully clear why, as it seems more of an emotional choice than a strictly logical one.
So, why do you prefer the OSR clone that you do?