Mine is defiantly Unreal Tournament. But to stay with the times, I'm rolling Halo Reach for me competitive needs. What about you?
I know they're both new but I think Monday Night Combat and AC:B rank up there pretty high, if for nothing else they do something different and pull it off incredibly well.
If we need to be digging back a bit deeper I'd say the original DotA mod - sooooo much fun.
Of all time? Command and Conquer Renegade had the best online I have ever played in a game and nothing has even come close to matching how fun it was, but since EA shut down the servers I can't play it any more.
I recently re-downloaded Bad Company 2 on Steam a few weeks ago and have been playing that every now and then lately, but I tend to go through a lot of hot and cold phases with that game. I will be really into it for a few weeks to a month, but then I hit a wall and lose all interest in it. I only enjoy playing the sniper class and I maxed out that class within the first few days I played the game. So now I don't have anything left to unlock and look forward to on the RPG side of that game.
I like to dabble in Medic from time to time, I love playing support in games and running around healing and reviving people in Bad Company, but I hate the medic gun so it doesn't last long.
For myself, and a number of years it was Star Wars Jedi Knight ; Jedi Academy. I still have the box and I hol dmany fond memories of many countless hours spent playing that game with the first clan I was even actively invloved in. The lightsaber combat was quite good in that game, with variations of "light", "medium", and heavy" saber strikes, and damage localization, (at least it seemed like it to me), saber strikes to the head seemed to always count as a one shoot kill. Man I loved that game, still do.