Fergie's rivalry with them, and their constant bating of us and unsettling of our players in the newspaper they owned.My dad taught me to hate Bolton. He wasn't keen on Burnley either but that was just Bob Lord. As for Real Madrid, I don't understand the hatred from United fans. After Munich, they helped keep us afloat.
I don't hate them like I do Liverpool (think they also helped us tbf), but there's definitely some rivalry there. Maybe less so now Fergie's gone.