Lately for me it's been CoD4... I have about 10 1/2 days logged in on just multiplayer so far.

All time top 5... geez i haven't a clue in the world... but off the top of my head...

1: TETRIS!!! Seriously, how can this game NOT be on everyones list??? 1000+ hours lifetime easily

2: Blades of Steel/NHL '94 (i was and still am a hockey fanatic, upwards of 500 hours total)

3: Super Mario Kart (maybe just as much.. minimum 400-500 hours total)

Edit:  10.5 days = roughly 250 hours, so stickin COD4 here so as not to look dumb

4: Super Smash Bros Brawl. (close to 200 hours total playing time)

5: Mario 3 (replayed and beaten it countless times... 150- 200 hrs)

Final Fantasy VI....at least +1500 hours of my life.

But I fell very proud about it,and yes,I wil do it again :)
Perhaps when the remake come? ;)

Melee.

the lost in blue series.

WoW followed by FFXI.

As for non-MMOs, that would be SSBM.

final fantasy game made by nomura.

over 100 hours each

starcraft

mario kart

I'll just lump FFs in one big pot as I couldn't tell you which one of them since 6 has taken most my lifetime hours playing, played most at least 5 times through, FF7 and FF8 a few times more, so maybe those.