I have played many multi-player games in my time, some off-line and most on-line.  A sign of a good multi-player game is that you dream of playing it when your not playing it.  Back in the N64 days I used to be hooked to Golden Eye and Mario Kart 64 multi-player with my mates and though nothing would ever beat golden eye.  But since getting a PC and going online I have changed my mind somewhat.

I also used to play Delta Force (PC) a lot, I must of put 100's of hours into that game, plus I used to be a master at Half-life Deathmatch back in the day.  I have also played a lot of MMO's but I don't think these type of games really count as mostly your soloing.  I have also played Track and Field over Xbox Live with other people (must say great fun indeed).  So really for me 2 games do stand out from the crowd... Counter Strike and Battlefield 1942.

Both games are really good online or on a Lan and I have had a lot of fun out of both.  I used to go to LAN events years ago with like 40-50 other people and then the game people played were mostly Counter Strike and battlefield (must say some of my best gaming days).

So I must go with Counter Strike as my best multi-player game ever, even if I not played it for some time now, it is still one of the best games out there.

Call of Duty 4.

Goldeneye.

It hasn't aged well and is hard to play today. But there has been nothing that has come close to the amount of fun I had with that game when it was new.

TNMT: Turtles in time, Streets or Rage, Knuckles Chaotix, Mario Kart, Contra

Call of Duty 4

Brawl or Goldeneye

World of Warcraft, Starcraft or Timesplitters 2.

Mario Kart 64 or Goldeneye.

Call of Duty 4: Modern Warfare

Wacraft 2 Kali days.