As a healer, it doesn't make much of a difference as long as they know how to manage threat and cooldowns when needed.
As a tank, my druid is always my favorite even after the dmg/cd nerfs, DK is a pretty fun taking style and Warrior is the most fun with the most buttons to press, haven't done a paladin tank yet
my main is a dk tank and i regularly finish bosses after healers die. imo dks have about the same self healing capability that paladins do. plus the mastery is amazing.