for me it was elder scrolls 4:oblivion , and the reason is i love being able to do what ever i wanted , had tons of stuff to do,loved the fps view,liked the dlc u could get(mehune razor),loved the graphics,all the choices u could make and the problems that came with each action u made.

Kingdom Hearts 2

Bionic Commando Rearmed, simply brilliant!

pokemon gold

Super Mario Bros.
The Legend Of Zelda: Ocarina Of Time
The Legend Of Zelda: Majora's Mask

I've played Brawl the most. I've beaten SMG 3 times, so I guess I beat that the most.

Mass Effect, with Fire Emblem Radiant Dawn coming in second.

Metroid Prime 3: Corruption.
I played a lot more Brawl though.
And also slowly beating SMG.

Excluding FF X, which I spent 96 hours on earlier this year

Lost Odyssey, nearly 70 hours, and I still have a few things to do

I have beaten Assassin's Creed the most, I did it 3 times, 2x on Xbox, 1x on PS3

ooh

Dead rising... completed 10 times (hard to get 1000 G's in anything less than 6 playthroughs)

Gears of war....... completed 8 times....... (4 times for achievements, 4 times for fun)

XBLA...... streets of rage 2..... completed over 50 times (to go along with the 100+ times i completed it back in the day on the Sega megadrive)