Hmm...Well...
Obviously #1 would have to be JJ'
and the rest in no particular order...
(As it varies from map and classes)
Darthwiley...Sniper mostly
Ace...Point Scoring
KennyUBastard...Recon
Sho'Nuff...Varies
Vash...Varies
Mr. Noob...Varies
SWL Legend...Varies
SWL Russel...Recon
Balthazar...Varies
There's others but for me these are the toughest to kill