If we were talking tBC times, priest easily. Practically go oom after 2-3 mobs and the dmg was honestly shit. That along with being squishy as hell with no easy way to kite/cc. Lvling a priest to 70 was mentally suicidal, I was practically scrounging for any grps wanting to run instances.
ATM with all classes being easy and speedy as hell to lv, I give it to rogue due to lack of consistent gap closer as melee(from what ive seen) and the need sometimes to stealth and go behind enemies.