Team Fortress Classic. Started playing in late 2003. Since then, I have well over 1500 hours recorded in the game. I still play it on a regular basis. I know by the time I'm dead, this will definitely still be the most played game I have.
Half-Life 2 (three playthroughs)
Knights of the Old Republic (two 30 hour playthroughs)
Mass Effect (two 30 hour playthroughs)
Modern Warfare 2 (yeah, like ~150 hours just online on my own console in the last five months)
NHL 09/10 (don't even know the amount of time spent playing these)
call of duty 4: modern warfare (a ton of online time spent)
killzone 2 (a crapload of online time spent)
demon's souls (beat the game probably around 20 times. very addicting)
diablo 2 (went back to playing it after a 5 year break. had to try out some of the new things i missed ...ie, uber trist, new runewords, etc. got hooked on it again for a couple months. i stopped because of the recent ladder reset. couldn't be bothered gaining back all the wealth again. kinda glad they reset the ladder because its too damn addictive. i fear diablo 3.)
close but not quite on par with the time spent on the ones mentioned above -
Left 4 Dead 1 & 2
Battlefield: Bad Company 2
the most of all time = by far Diablo 2. i have never played any game more than this. nothing comes remotely close.
2004-2008 WoW In 4 years I had about 375 days played on my main character. Yes I was addicted lol.
2008-present Nhl 09, NHL 10, Gears of War 1/2, COD MW 1/2, Mass Effect 1/2 and then Fallout 3
I played a lot of WoW in the past as well (not that much since that's ~25% of your day everyday ) but I never liked the numbers they gave you because most of your time when you hit the level cap, you're just afk waiting for dungeons/raids/pvp matches. I think by the time I hit the level cap, after about 2-3 weeks to catch up gear-wise for raids, I was afk probably 40-50% of the time.
TF2. I reset my stats once about a year or two ago. I'm rapidly approaching 900 hours on this current stretch.
Prior to that I put in probably a couple hundred hours into both CS:S and DoD:S.
Diablo 2 probably had a couple hundred hours.
Sadly, Minesweeper in college was a big one. Hours of minesweeper while listening to music when I should have been reading/studying/writing papers. It still felt like "doing homework" if I didn't load a "real" game.
The Need for Speed series (up to Underground).
GTA III and beyond. I managed 100% completion for Vice City. Twice.
Unless I'm missing something obvious from my past, those are probably the only ones that crack 100+ hours.