Liverpool.
They honestly believe that they are the only real football club in the world, the only club with passionate fans, the only club with legends who played for the badge, the only club with deserved pedigree and a tragic history and a good atmosphere in the stadium when in reality they just sing You'll Never Walk Alone twice a game without stopping and thinking for a second what the lyrics actually mean. They look down their noses at most everyone else because they are God's Own Club™ and they are simply having a short break from dominating Europe. And when one of their players, a young goalkeeper thrust into a situation where he's clearly out of his depth as a result of the club's poor foresight, makes a mistake? They turn on him like a pack of hungry wolves.
United hate us. Liverpool hate everyone.