I would say DKs.
Not to say that DKs are a bad class - I honestly don't know, but all the DKs I do know are either quite hard to heal or very bad DPS (which I assume is because they're holding onto all their resources until they take damage). I think it's the fact that because they start at a higher level, people wanting to try a tank class go for them and never actually learn how to play while levelling up because every dungeon can be won just spamming Heart Strike. They never really learn the nuances of Death Strike and runes.
Paladins and maybe Warriors have the best rep. They're quite tough even if they don't know what they're doing and they wont die in seconds like a badly played Monk will. You can definitely notice the difference between a bad and good Paladin though.