The list below only applies when they're haxing, of course.
Demoman - pipski, wtf-lah
Medic - Tem (rarely see her anymore), Yoshi, kitty, and any other regular I play with and call a haxor/spy.
Engineer - Avalanche
Sniper - Jobo, Starraven, Bloodsire (When he plays. I think it's down to once a week now.)
Soldier - Pink
Spy - Prep H, Liwi, Seraphim
Scout - Nebz
Heavy - RavinReverend, Avalanche
Pyro - Can't think of any atm as several noobs have been playing this class since the pyro achievements were released.
Luigi goes up there somewhere but forgot what he plays. =P