I would have to say Metal Gear Solid 4 because I enjoyed playing Snake and being able to use all those cool weapons.

Going by play time: skate 2

Super Smash Bros Brawl probably.

Valkyria Chronicles, give it trophies and I'll die a happy bunny.

MGS4 does indeed have awesome weapons.

But my L4D is my baby :)

Do DS games count?

I would say:

1.)Megaman ZX -- Play time: took me around 15 hours to do everything I wanted.
2.)Ninja Gaiden Sigma -- Play time: 90 hours
3.)Tales of Vesperia -- Play time: I'm at 115 hours right now.
4.)Street Fighter 4 -- Play time: Unknown. Probably 15-20 hours.
5.)Super Paper Mario -- Play time: 35 hours.

Others I liked:
Ninja Gaiden 2 -- Play time; 20 hours.
Megaman ZX 2 -- Play time: 8 hours.
Digimon World DS -- Play time: 40 hours.
Devil May Cry 4 -- Play time: 17 hours.
Zelda Wii -- Play time 48 hours.
Mario Galaxy -- 40 hours.
Ratchet PS3 -- I don't know
Dragon Quest V -- Over 20 hours right now.

so far it's been..

Tales of Symphonia: Dawn of the New World

followed closely by Tales of Vesperia and The Legend of Zelda: Twilight Princess.

You liked symphonia 2 better than Vesperia?

viva pinata is the greatest game of all time so i have to say its my favorite

Would it be funny if i said Killzone 2? lol