I like MP games that take a good deal of skill and depth to master. MP games with rigid competition and normally no true cop-out easy way to win I guess.
For shooters, Quake, UT2k4, and CS1.6 are the best damn shooters I have ever played. Fast paced, intense, tactical, and chaotic. I could play these games forever, they just never get old for me; though I have to give Halo a shout out as well. There have been something lost in shooters today that these games had which made them stand out and special. I honestly hate the direction most shooters today have taken, though there are still a few shining beacons out there.
Fighting games are probably these best competitive MP I've ever experienced, even despite the rage they will induce at times. Of course the fighters that stand out for me are Arc system works very own Guilty Gear and Blazblue. Fast, intense, bone crushing depth, excellent timeless gameplay. Guilty Gear is the best fighting game ever for me, and I can't wait till I get my grubby mits on Accent Core R and start kicking ass with Chipp and Baiken again. A little cautious about BBCP and some of its changes, but we shall see; I will continue to obliterate all with Hakumen - the white void.
I have made plenty of friends, rivals, and black listed people over my time playing fighters, and it's always fun getting a room up for us and playing for hours to see what bullshit come back one of us makes next "p