What's the must played multiplayer games?

For me:

1) Counter-Strike Source/1.6

2) Left 4 Dead 2

3) Modern Warfare 2

What say you?

TF2 and Bad Company 2. I've been playing single player games a lot lately.

CSS, and League of Legends for me.. haven't played many PC games so I'm not too sure about other must-play multiplayers

TF2 and CS

ETQW, Borderlands, COD2, WAW

Battlefield 2
Project Reality

Day of Defeat is pretty good. Used to be more popular so there were more map options

Warcraft III up until a year or two ago (it seems to have died off now)

Star Wars Jedi Knight II: Jedi Outcast at it's time.

edit: Wait... Is this thread asking "What's your most played multiplayer games?", "What's the most played multiplayer games?" or "What are the must play multiplayer games?" :?

1.L4D2

2.TF2

3.Battlefield BC2

or Killing floor