Hmm... thinking on this for a bit, I have to say Persona 3:FES. Yes, P4 is a better game, but it was FES that I got to first, and it's what made me a raving Atlus fanboy. It was the game that showed me that yes, there are still games out there that focus on a good game and challenge. I've been going back through more of Atlus' catalog, finding more and more gems. And I'dve not found them if it weren't for FES.
-dunno001
-On a quest for the truly perfect game; I don't think it exists...