IMO Q2 has the best multiplayer of any game. Best weapons, user created models and maps etc. People seem to be into realistic weapons now, but the weapons, speed, can gameplay in Q2 has yet to me matched. Rocket Arena 2 and Chaos rocked.
Never finished either in single player, though I feel Unreal had some cool moments, like the first time you run into that Dreadlocked dude in the hallway, or when you see the Titan (or whatever the giant was called) for the first time and he chucks boulders at you.
If you want something old school to play, check out SiN!!!!!!!!!!!!!!!!!!!!!!!
.....just patch it up first.