In raw numbers, almost certainly multiplayer games. I don't actually know most of the figures except TF2.
Team Fortress 2, 400 hours: Because it's a ton of fun, entertaining, has a meta-game with items and stuff and is regularly updated so you always have cool new content coming up.
Call of Duty 4: Modern Warfare: Shooting. Lots of people-shooting. I loved the Search and Destroy missions in particular. Most other gametypes, particularly FFA and TDM were a mess, but S&D is golden, particularly being the sole survivor in your team and stealing all the spotlight by turning things around.
Unreal Tournament 2004: It's Unreal Tournament! It's shooting like crazy in crazy environments! Like CoD4, I played one gametype almost exlclusively, and this being UT2004's quite honestly revolutionary Onslaught mode. A gigantic sandbox with vehicles rolling everywhere, impromptu camaraderie, cheeky sniping, raptor dogfights, and of course, going straight up above the tower of Torlan to grab that Redeemer.
Civilization IV: Because one does not simply play a couple hours of Civ. There's little to explain in this regard, really. You start playing Civ, you spend hundreds of hours making civilizations, fighting opponents, doing cool stuff. It's a shame Civ V is so utterly boring to play - I only ever finished one game of Civ V, and on realising that it didn't even have post-game charts, was never bothered to finish any of my next games.
Age of Empires 2: Gave me a love for history and is the sole contributing factor to my knowing anything about medieval history. I wasn't into any strategy groups or anything, so I didn't know how competitive the RTS field can be, but what I mostly liked was building stuff up. I was prudent enough to build armies, defend and attack, but what I always played for, was starting with a Town Square and a couple of blokes, and the ending the game with a gigantic, cool city.
Jedi Knight: Jedi Academy: LIGHTSABRE SLASH FUN TIME. Played through the campaign quite a few times (the modular game design was great), and the multiplayer quite a bit too. In restrospect, JKA may have been the worst multi I played, not because of the design, but because of that horried JKA+ mod that turned every server into a private authoritarian dystopia.