I don't like Kel'thuzad on Heroic. The permanent steal on his hero power is a bit much.
My least liked would probably be Noth (at the time I did it) and Maloriak. Noth's plague card is stupidly op in a matchup versus any creature deck, and when running creatureless I kept getting foiled 1 turn before lethal by
Ice Block. This was back in the day. Now he's much easier.
Maloriak has 3 3/3 charge minions for 2 mana in his deck. <- If he does this early, the game is just lost. I don't think there is a come back mechanic, that I know of, that currently lets you win this fight after he does that. I've at least seen people win the Drakkisath fight without deathwing.