Actually I think the one that kills me the most are mages.
How to piss off a multiboxer with a mage:
Step 1. use invisibility to sneak up on mages
Step 2. Frost nova to nicely glue toons in place
Step 3. Important - move behind all toons - this way only the main can turn
Step 4. Use arcane explosion, cone of cold, etc... until boxer is well done
Step 5. When toons are free, they turn to kill you, blink behind them
Step 6. Pop all cool downs for spell damage if needed or sheep main and run
Step 7. If steps 1-6 are done correctly - dance and lol on 5 dead shammies

I think hunters are the funnest to kill... Usually they stand at a distance, send pet, begin Dps... I do the same with an earth elemental, I win.
There was 1 hunter though... whos sole goal in the AV was to kill my totems... it was annoying.