A very hard list to make. But I'll give it a try:
10. Call of Duty Modern Warfare 2
9. GTA:IV
8. Every other Call of Duty game
7. Halo 3 and Halo Reach (not sure about ODST and 2)
6. IV:MP
5. Garry's Mod
4. Battlefield: Bad Company 2
3. Left 4 Dead and Left 4 Dead 2
2. Minecraft (laggy multiplayer though, still fun)
1. Team Fortress 2 - the absolute king of multiplayer First Person Shooters, if not the king of multiplayer games in general; everything feels so right! With the exception of the hat-addicted community and the microtransaction store... even though that you can get items you buy at the store by other methods, like the drop system and trading with people. Nonetheless, it's 15 euros well spent. Just ignore the asses that brag about their hat collection and you should be fine.
Here's a list of other multiplayer games that aren't included in the list, but you should give them a try if you want:
Magicka, Battlefield 2, Company of Heroes, all Counter-Strike games, GRID, Worms Armageddon and Reloaded, Killing Floor, Trackmania Nations Forever (free version of Trackmania United Forever), Half-Life 2: Deathmatch, Day of Defeat: Source, VC:MP (Vice City: Multiplayer, Argonath hosts a server there), MTA:VC (Multi Theft Auto: Vice City, same deal as VC:MP), etc...