Bioshock... but then they had to go and add more, didn't they? Bastards.
My second full grand game was my favourite though: Dead Space. That was so much fun I got all the points without even thinking about it. I've played through it 4 times and if I hadn't given it to a mate to play I'd probably have another crack. Damn, I loved that game.
Mine was Oblivion, I sunk over 400 hours into that damn game, on two characters, too. Then after that Rainbow Six Vegas Two, then Bioshock, then Resident Evil 4, then Fallout 3, then Street Fighter