It obviously depends. But generally the best farm characters are:
For low level content: the characters that move the fastest (ferals, hunters, rogues)
For hard content: Monk/Guardian/BloodDK/ProtPal
For AE pulling: Same as hard content imo. But can be done extremely efficiently if you know how to do an advanced profile with any class that can AE hard . However making an advanced profile is no picnic.