Another vote for the UT's here. UT99, starting off against bots, and the game will ramp up the difficulty as you get better, it's a great way to learn the ropes.
It also depends on what you want to do, if you want to improve your reactions, insta-gib games will really help for example.
It also depends on what you want to do, if you want to improve your reactions, insta-gib games will really help for example.