Under Ferguson's stewardship United won the Premier League 13 times but have yet to triumph in the division since his departure in 2013, while arch rivals Liverpool ended their 30-year wait to win the league title again under Jurgen Klopp last summer.
It has created a debate about a changing of the guard and the Scot conceded he would not fancy taking on the current best side in England.
"They are the two most successful teams in Britain when you add up all the trophies," Ferguson said on 'A Team Talk With Legends' panel in aid of Sports United Against Dementia and the League Managers Association's charity In The Game.
"United against Liverpool, it is the game of the season - the most successful sides in the country.
"Thank goodness I've retired, because when I see Liverpool's performances over the last two seasons - they have been phenomenal."
Additional reporting by PA.