Sonic's Ultimate Genesis Collection, since I'm a retro gamer.

Probably Skyrim, because it was huge. If it's a series it'll be definitely Fifa. Have played easily over 200 hours across 5 games.

SP = God of War III. Played it so many times, 100+ hours on that game.

MP = God of War: Ascension or The Last of Us. Spent a lot of time on both.

360 is my main console, so this shouldn't be much of a surprise. Also I don't really limit myself with playing only a couple of games each gen.
I share my library with my elder brother and a couple of other friends, so I've played hundreds of last gen games of which I completed 232 of them (and still counting).
my record for last gen would probably be Pro Evolution Soccer 6 (PC) with ~400 hours followed by Skyrim (360) with 120 hours.

MGS Peace Walker. I got 300 hours easily. And 200 hours on another save I lost formatting the HDD.

And 360 hours on GTA Online.

I've been trying to think through this in my head

CoD 4, 50+ hours

FIFA 12 or 13, I lose count, but a serious serious amount of time

GTA IV, 60-70 hours
GTA V 50 hours

Uncharted 2 maybe 70 hours, about 5 or 6 playthroughs with some online multiplayer thrown in

Kingdom Hearts I.5 HD Remix, I think I spent like 80 hours through my 3 playthroughs if I add it all up, worth it for the platinum

Ni No Kuni probably wins for now with it's 80 hours

Technically, FF X wins since I spent 130 hours playing it via BC, but give it a month or 2 and FF X HD will be the winner. I'm like 15 hours through now {that includes Vita playtime though}

* Persona 4 Golden is currently sitting on about 110 hours on Vita

EDIT: Skyrim with 60 hours and there are probably some others that I have forgotten...

I'd guess Rock Band 2 or FIFA 11 due to playing with friends for various evenings.

On my own hard to say, maybe Disgaea 3.

Same here.  I think I've mentioned on this very site that it was the best PSP game on the market!  Then, when it was available for Vita (I got it digitally), I transferred my games and save data to the PS3 and then to the Vita.  I mistook "Game Data" for "Save Data" or the other way around and deleted my entire game!  I've never been more disgusted with myself in my entire life....and I once ______ at _____'s  _______!!

It's between mgs4, red dead redemption, and killzone 3.