It’s a simple question that has prompted a thoughtful response in recent days as Reds supporters bid farewell to Rio Ferdinand and Nemanja Vidic, two men whose defensive partnership laid the foundation for one of the most successful eras ever seen at Old Trafford.
Their alliance is often compared to those of yesteryear and was preceded by the short but impactful reign of Jaap Stam and Ronny Johnsen, who formed an axis that underpinned the club's incredible Treble success in 1998/99.
The early-Nineties pairing of Steve Bruce and Gary Pallister is another that can lay claim to superiority over the others, given how their defensive prowess helped United to dominate English football under Alex Ferguson.
Older fans will also remember the impressive endeavours of Martin Buchan and Gordon McQueen, a pair of defenders with contrasting styles who guided the Reds through the tumultuous late Seventies and early Eighties.
All of these partnership had their merits but as for which one deserves to be labelled the greatest, we need your help, please, to decide.
Vote: Which is United's best centre-back pairing? >