#1 Matt Busby
Manchester United fans, look away now!
Yes, legendary United manager Sir Matt Busby played for both of the Red Devils' bitter rivals. In fact, those were the only two clubs (officially recognised) he played for. Busby started his career at Manchester City in 1928 and stayed there for eight years, playing a key part and winning the 1934 FA Cup.
The Man United legend joined Liverpool for £8000 and made the number 4 shirt his own at Anfield. His consistency in midfield earned him the captain's armband. Busby was even offered the role of assistant coach at Liverpool, but a disagreement in views meant that he would go elsewhere for management. He joined United and the rest is history.
It is quite the fact that one of the biggest legends in Manchester United history played football for both Manchester City and Liverpool. Truly mind-blowing.