Say what you want about Just Dance, its fun and we enjoy thats what matters. It is kind of how you can just waggle on some games. Just because you can does not mean you should. It is funner to actually play it.

Online counts too if they are your friends. Guess I should have made myself clear as many people do only play online with friends.

LittleBigPlanet
Army of Two games

Call of Duty 4: Modern Warfare

1. LittleBigPlanet!
.
.
2. Mario Kart Wii
.
.
3. New Super Mario Bros.
4. Killzone 2
5. Left 4 Dead 2

If you're talking strictly local mutiplayer, it'd be more like:

1. LittleBigPlanet
2. Mario Kart Wii
3. New Super Mario Bros.
4. Super Smash Bros. Brawl
5. Super Stardust HD

Edit:

Suppose I should also give a nod to Demon's Souls.  It's online system is definitely designed to prevent you from playing with people you know, but I've somehow managed to play that game with friends more often than most others on the above lists.

Hmm
pixel junk eden for a while
Rock Band for a longer while.
Resistance 2 for not very long
Wii Sports with family
Rabbid games with family
LBP a bit with friends
Demon's Souls a lot till I got the plat and my ps3 died
GTA4 recently

Coop games I love                                                                

COD 5 (No 4 or 6 something is really wrong with that)
Halo 1,2,3, ODST
Borderlands (My all time favorite coop game, the longest including expansions)
Haze
Resistance: Fall of Man
Trine
LittleBig Planet
Castle Crashers
Gears of War 1, barely 2

Left 4 Dead 1, 2

Resident Evil 5

Mario Party

Online multiplayer

Halo 2..3

COD 6,5,....4

Resistance: Fall of Man

Mario Kart

Grand Teft Auto IV

This list makes me relize almost 100% of the 360 gaming I do are social gaming! Microsoft is on a good path in this respect for me, and I hardly ever give them props.

Left 4 Dead 2 and Gears of War 2.

Singstar, Brawl and LBP.

Mario Kart Wii

Gears of War

New Super Mario Bros Wii

Super Smash Bros Brawl

Tatsunoko vs. Capcom

Petz: Horsez