What's your favorite competitive multiplayer game?

Mine is defiantly Unreal Tournament. But to stay with the times, I'm rolling Halo Reach for me competitive needs. What about you? 

Modern Warfare 2.
 
I tend to stick to the Single-player modes in most games I play. But I spend a lot of time with MW2, and I had a lot of fun with it.

Call Of Duty 4, though I just bought Bad Company 2 so that might still change.

World of Warcraft. PvP in WoW got a lot of depth.

GoldenEye 007. 
 
 
Or, I'm gonna throw in a darkhorse and say Lead & Gold. I'm not sure how people felt about it, but I thoroughly enjoyed it. Not the deepest or the most variety, but I had a lot of fun with it.

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.

Counter Strike and Guild Wars.

If Killzone 3 is better than Killzone 2 than I would have to say that, played the beta and loved what I saw so far. As for a game that is actually out now I would say Uncharted 2.

Probably TF2, Uncharted 2, and maybe gears, but my list usually changes from time to time depending on the mood.

Fat Princess or Team Fortress 2. I play a fair bit of Halo Reach and Bad Company 2, but I find it hard to take competitive games seriously in general.

I used to play GW PvP a bit, and I really enjoy the AC:B multiplayer (even though matchmaking is very close to being unusable) but the one I keep going back to every now and then is Halo: Reach

AC brotherhood

I guess Assassin's Creed: Brotherhood, because it's the only competitive multiplayer game I've played that doesn't usually end in me yelling at the TV and shutting off the Xbox with the desire to snap the disc in half.

Bad Company 2 for sure

Unreal Championship 2. 
 
God that game was insanely fun.

" Unreal Tournament  "

I wish I could play Bad Company 2 more often. ):

Starcraft 2 for sure never played a game this competitively before.

Currently StarCraft 2. I haven't played in awhile though. 

The best are:
Team Fortress
Gears of War 1
Starcraft
 
Gears of War was the best ever for me, but its time briefer, as the feeling of 1 did NOT translate to 2.  Damn smoke/conc nades and plantable mines ruined it, that and they made Annex execution rules.....so dumb.

Fuck all yall Dota FTW! :D

I'd say BC2 if I had a squad to play with, but since I'm usually a loner, Black Ops is it.

I don't have one at the moment. I'm really pulling for BRINK to get me back into competitive multiplayer. If not, I might be doomed. 
 
SOCOM II, CounterStrike: Source, and Metal Gear Online 2 have been my favorite competitive multiplayer games from my past.

Call of Duty 4 and Gears (1&2). 
 
Back in the day is Quake 3; I actually do hop on Quake Live from time to time and I always have a blast on that.

Smash Brothers, any of them were ace to play versus.

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.

Halo1 on the PC. Though laggy as hell, it's addictive.
 
Currently its Call of Duty Black Ops. Reach was horrible.

Halo Reach

Starcraft 2
From other popular MP genres, I say Team fortress 2, DoTA "or LoL" and Call of Duty 2. Yes CoD2, I still play it quite a bit and prefer it over it's "modern" counterparts.

Team Fortress 2 without a doubt.

Halo 2. I played a lot of effin Halo 2 back in the day. Man, those were some fun times.

I've put the most hours into Starcraft, Tribes Vengeance, Counter Strike Source, and Gears of War

SOCOM II

WoW and TF2

StarCraft II  
 
I would have said Ghost Recon but that was way back when and before they introduced third person and screwed the multiplayer up.

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.

Uncharted 2.

StarCraft 2, it's the only competetive multiplayer game i've played for more than 3 months. and it's still very compelling.

Man, I miss playing UT2004. Bombing Run was an awesome mode. 
 
Besides that: Ghost Recon 1, BF2, and L4D are some of the most fun I've had playing online.

I really want to say Borderlands but I'm pretty sure that doesn't really count. 
 
Hmmmm, I'd probably have to say.....Call Of Duty 4.

" Halo 2. I played a lot of effin Halo 2 back in the day. Man, those were some fun times. "

Bad Company 2/ COD 4.

Unreal tournament 2004 without a doubt, but within the last few years probably Halo 3.

Fat Princess.

Team Fortress 2 by a large margin. 
 
I might take a break from it now and then but it's always great when I return to it.

Uncharted 2 and Resistance 2