Not an United fan, but if I were a supporter, I'd say Liverpool. A Man Utd fan definitely doesn't want Liverpool to win a league title, that would mean that Pool will get closer to Manchester United in terms of league titles won (right now: Liverpool 18, Man Utd 20). And if one day, Pool gets to win two or three more league titles, there'll be no debate as to which club is the most succesful in both english football and european competitions. I think in terms of intensity, the rivalry with Liverpool can't be compared with a local derby, hence why I think United fans hate Liverpool a lot more than City.