I think it's all on play style and how long the server has been going and the time limit in the server. If it's short, I'll play with trunks, because he ascends fast...when he ascends, he's much better then buu. But while he isn't ascended, he is very much at a disadvantage.
If I want to melee, I usually go with vegeta, because he usually responds to the melee key the fastest
I like buu and cell pretty much, but I tend not to play with them cause I like playing with the average characters the most. (if someone comes on the server and start owning everyone, I'll usually switch to one of them unless I'm vegeta transed). Plus, turning people into food and regaining health is pretty nice.
Goku I like using if I want to do alot of huge damage early in a server. Spirt bomb is worth it. Please he responds fastest to block, so if someone is comming in at me, I can easily abort and block quickly, then switch to melee.
Gohan, Krillin, Frieza and Piccolo are horrible in my eyes. I don't like using them cause thee are much better characters to choose from. Gohan's speed is alright, but with his low health he's not worth it. If I want discs Ill use Cell's. Same with SBC. Piccolo's scatter beam is alright and it's like a faster version of kame torpedoes, but again, it's not worth it.