I'm gonna have to say Call of Duty 4. Up until a few months ago, my blood pressure would spike like no other whenever I got killed by cheap crap like martyrdom, frag x3, last stand, M16s, etc. But after putting about 16 days worth of time into the multiplayer, none of those game breaking perks really bother me anymore. At the end of the day, no matter how random and unbalanced it might be, Call of Duty 4 is one shooter that will forever stand out to me as being one incredible multiplayer experience.
Runner up would have to go to either Quake Live or EverQuest 2.