Erebus, since he is the biggest PITA to fight against, and both good at running away, and going in for the opportune kill. He is very reliant on the right minions and gear at the right points in the game, although that goes for Rook as well, but I like my Erebus *mobility* (teleport 25 times per game or so, use speed items -always on the move, if not misting a chokepoint).
