I'd say hockey, but I've played that more than others, so my views may be biased. The pace is fast, there is a lot to watch, and many penalties are very subjective. It isn't as bad as soccer, but the diving in hockey happens all the time too, which makes it that much harder on the refs.
It also seems like hockey refs take a lot more shit from coaches and parents, but the culture of fighting in hockey might have something to do with that. Call too many penalties and the refs stole the game from the players. Call too few penalties and they let the game get out of control. They just can't win sometimes. It seems like refs in other sports have to deal far less with the accusation of "Letting the game get out of control".
Even playing beer league hockey, I chuckle at the abuse some guys hurl at the refs. I'm just glad they are there and we get to actually play, but some players treat it like they have a pro contract on the line.