What's the best RPG, and why?

It could be argued that there is no "best" RPG, as different players enjoy different sorts of games. However, it's pretty much universally accepted that Rifts is a horrible game, and if that's the case then there must be some quantifiable element of quality in RPGs.

I'm personally inclined to go with the obvious, D&D, but that would inevitably yield the question of which edition, and that pisses everybody off.