Actually, I think there have been a few games that have come pretty close to my definition of perfect (I won't actually define it, so try to hazard a guess based on the games I list, if you really must)... including: Portal (naturally), Left 4 Dead (it is the only online multiplayer game that I've played that was actually fun), Shadow of the Colossus, Silent Hill 2, Mass Effect, Assassin's Creed (okay, this one is the furthest one down from my idea of perfection, but it gets enough things right to be on this list), and Day of the Tentacle.