I don't think there will ever be a player that makes me angry more than Fellaini. Its not actually him (I always liked his hair) but everything he represents. That awful botched 1st transfer window post Fergie (that was just a warning of the mess that was to come).
Mainly its the way teams played when he was on the pitch. We basically became the team hoofed it to Fellaini and hoped for a knock on. If that was Plan B, I probably would have be fine with it, but for huge periods under Moyes, Van Gaal and Mourinho it was our plan A. You would expect it from Moyes and Mourinho, but Fellaini made a Van Gaal one a renowned possession based managers ever, with a team full of players like Di Maria, RVP, Rooney, Falcao, Blind, Herrera, Schweinsteiger, into a Sam Allardyce team from the 90's.