Over the last 40 years who has been Man United's greatest rivals.......?

It has to be Liverpool, at least for the 12years i've been wacthin football lol..im just 19yrs old. My reason is that Liverpool have won the league about 19 or 17 times not so sure, and Manutd is right behind them. Last season Manutd reduced this to just one less than Liverpool. And Manutd vs Liverpool matches are always tense cos Benitez has never won against Manutd since being Liverpool manager