I'll keep this short and sweet: what's yours? What's that one sleeper hit you just can't get out of your head?
My vote is for SaGa Frontier II for PS1. The artwork combined with an awesome story to give the game a truly unique feel. The battle system kicked ***- nice and difficult. And I still can't get the theme song out of my head!
Qualifier: Xenogears was NOT a sleeper hit. :D