As everyone know, there are a lot of things that makes game peoples favourite game for multiplayer playing. Of course one of the most important thing is how many people are playing online in that game, because even if we think that some game is great for multiplayer, and there won`t be other people who thinks this same, we can play...at most with bots.
So how do you think, which game is most fun game to play multiplayer? Which one is the best, that makes you sit and play by hours, and makes your family begging you to eat something, because you can die without food:P
My choice:
Counter Strike 1.6 and Counter Strike Source
And i keep in mind that GTA is great multiplayer game too ;)

Team Fortress 2, Killing Floor (UT2004 zombie mod), Doom 2 with custom wads, Civilization 4... but no CS for me :P

Sins of a Solar Empire

Quake 3 Arena of course. The game may be dated, but even after 9 years. The game is still being played by many old and new players alike. I say, if a game that's been around that long and people are still playing it today. It has to be one of thee best multiplayer games.
Also for those who are too cheap to buy the game or don't want to invest money in buying such an old game, there's gonna be a free web based version (ad supported of course :P):
[url=]http://quakelive.com/[/url]

Don't play much online multiplayer (!), but Day Of Defeat (GoldSrc) is my favorite so far.
Way better than CS.

Ahh yes....Quake 3 Arena, this is classic! I still remember how we were playing it with my firends. Now not so much, but you have right, its one of the best.

Team Fortress 2, Diablo 2 (back in the day, I had to quit last year because of asshats), Doom II with Skulltag, and various half life mods.

RTCW: Enemy Territory
This is the one i like bes to play online.

Damn, how could I have forgot Quake III Arena? I spend hours on CPMA.

There was nothing better than you and 3 mates playing Goldeneye, splitscreen, on a big TV.
I spent a lot of time on CS but these days it's all about Team Fortress 2.

My vote goes to Team Fortress 2, Quake 2 (played it more than 3) and my new found love Multiwinia (great times to be had there).

SWAT 3 & 4, co-op. A thing of beauty.

Perfect Dark was always fair as well, I wish Rare would get off their fat butts and re-release Perfect Dark N64 Live enabled.

4 Player Co-op on Castle Crasher on Xbox Live!