FPSs in general, only Portal and Team Fortress 2 have really grabbed my attention out of every FPS I've played, which isn't a lot.
More specifically, possibly Ocarina of Time. I really enjoyed that game personally, don't get me wrong, but lots of people on the internet are under the impression that it's some sort of Gaming Bible for the amount of praise it gets, every 3D Zelda game since for me has been far superior. Better graphics, better story, more gameplay ideas, more interesting world to explore, etc.