Probably Orange Box. I think I've gotten over 80 hours in TF2, Portal, EP2, and I'm still playing TF2 nearly every day.
LEFT4DEAD411.COM
Bet with disolitude: Left4Dead will have a higher Metacritic rating than Project Origin, 3 months after the second game's release. (hasn't been 3 months but it looks like I won :-p )