I could say Zelda OoT *groan*..... but I won't!
REZ is certainly a top 5 contender I think. I get so ridiculously addicted to this game sometimes, it's simple yet deep, relaxing and intense. The running man boss at the end of stage 4 with the sound cranked up is still one of the most impressive moments in videogame history for me. It's a near perfect game.
Civilization, such a classic game, the original blew me away and taught me graphics mean shit all when it comes to compelling gameplay.