I like halo for the fact that its really how fast you react and know how the hitboxes work.
To run up into a whole team and kill them all with just 2 nades and assault rifle is fucking rad.
Plus I cant put the sniper rifle down when I get it. Ill usually take my last 3 bullets and charge, going for noscopes.
Im a PvPer and havent found anything quite like Ultima Online, besides Halo.
In UO it all depended on team work to not die to ganks, hell me and my buddies used to 3 vs 15 all the time. Given the 3/4 of them werent mages or were on foot though, so it made things a little easier.
But I get the same feel from Halo. MW2 I just die, I try to be a sneaky fuck and end up getting killed by a knifer,camper or nader, im not too perceptive of the maps though.
That's a big must in FPSs know. the. maps.