It's a little tricky because there are a lot of possible answers:
- The one I've played the most? Probably AD&D but that was 25+ years ago!
- The one I've run the most? Probably D&D 3rd Edition.
- The one that flexed my creativity in memorable ways after all that AD&D time? Fantasy Hero!
- The one that put a strong new spin on an old game and that I ran quite a bit? D&D 4th Edition!
- Games I really like but haven't played much? Warhammer FRP! Reign! Arcana Evolved!
So it's not as simple a question for me as it might be for some, but in the end I'm going to go with the game I am running and playing the most right now:
There are a lot of ways one could knock Pathfinder but for me, right now, it just works. I like it, my players like it, and we're enjoying ourselves when we play it.
There are a lot of books for it - you don't need all of them.
There's are a lot of rules in it - you don't need all of them.
It assumes you will be using miniatures of some kind - you don't really have to.
Any game is a framework for having a good time and for us for now we are having a lot of fun with it. Honestly, it's been a lot more fun than I expected to have with this set of rules when we started down this path. Given that, we're going to ride it out for a long time I suspect.