Best Multiplayer Game

What is your favorite multiplayer video game? (just the multiplayer portion of the game, so not counting how much you enjoyed it in single player)

It's tough for me to choose, but I'd go with Mario Kart Wii. I really enjoy playing that game in multiplayer, and I still do. Animal Crossing multiplayer is fun too though.

EDIT: Also, Pikmin 3's Bingo Battles mode is amazing. I just found it to be way too short, I finished playing through the entire mode with somebody else in an hour or so. It was fun and a very interesting mode (mostly due to the chaos) but still overall too short...

Without a doubt Counter Strike : Global Offensive.  Its unlike any multi-player shooter I've ever seen.  Unlike the average Call of Duty which is SHOOT ALL THE THINGS BADADADADADADADADADADA HE'S DED, this requires so much more teamwork and strategies.  And the combat is just wonderful, with a wide range of weapons, there's many combat options, but the greatest part of combat is the recoil system.  Instead of other shooters where your shot will always land on your cross hair, in CSGO, every gun has its own independent spray pattern, meaning you must pull down on guns to be accurate.  And when you put everything together, it builds one key feature that sets it above other shooters.  In traditional shooters, there is an EXP system when you level up when you get kills.  But in that, your character is leveling up, not you.  In CSGO you become a better player through experience.  This is something that few FPSs can do, and truly gives a feeling of achievement for doing well.

Animal Crossing, Little Big Planet, & Pokemon. At least these three series have always been fun to play multiplayer for me. ^^

Mario Party is pretty high up there with me, as well as Smash Bros, though the only problem is that people tend to not want to play Mario Party and they'll only play a little bit of Smash.