CTF: This is a very tactical game, and the best way to play online.
Dom: This can be fun but ctf is better.
Tests: I love tests, they are very fun to play, I suck horribly in airboard tests though.
SP: Although the fun in this usually expires when finished, it's still fun.
Co-op: Usually played with LPS. I played EvilMike's pack in co-op, and found many things you can do that weren't intended to be done.
Race: I like good races. Races can be very tactical if made properly.