The Red Devils have sat at the top of the list for nine years in a row but Jose Mourinho's men are now worth $135 million more (€103m) at $3.3 billion (€2.429bn), while Sir Alex Ferguson's side sits second in the business magazine's most valuable club list.
Barcelona is perched comfortably in third - $565m (€434m) behind United but $1.274m (€978m) ahead of fourth-placed Arsenal - and rose 99 percent in value over 12 months.
Bayern Munich fills out the top five having made the Champions League final last season as well as finishing second in the Bundesliga. The German club's value is sure to rise further in next year's rankings with the added interest of Pep Guardiola as coach, not to mention its triumph in this season's Bundesliga and a potential treble of trophies.
AC Milan is the top-placed Serie A side in sixth and the highest-placed club to be valued below the 1 billion euro mark this year. However, it is also the only club in the top 10 which has gone down in value over the last 12 months.
Chelsea's Champions League-winning season saw the team maintain its place in the list and boost its value by 18 percent, not so contrasting to the team below it, Juventus, which saw its value rise by 17 percent after returning to Scudetto-lifting ways in 2012.
Manchester City rose from 13th to 9th in Forbes' list after winning the Premier League last season and qualifying for the Champions League for the second successive campaign.
An English team not faring as well is Liverpool, which was fourth in April 2008 but finds itself 10th five years later as it continues to suffer from not competing with the European elite.
Outside of the top 10 sits Europa League quarterfinalist Tottenham in 11th, Bundesliga side Schalke in 12th, Champions League semifinalists Borussia Dortmund in 13th, Italian giant Inter in 14th and Olympique Lyonnais in 15th.
The final five in the list are South America's sole entrant Corinthians in 16th, Walter Mazzarri's Napoli in 17th, German side Hamburg in 18th, Olympique de Marseille (which dropped 18 percent in value, the biggest drop in the top 20) in 19th and Newcastle United perched 20th.
Interestingly, despite being considered one of the richest clubs in the world, Paris Saint-Germain fails to make the top 20, while La Liga's top-three duo are the only Spanish sides to make the list.
Last year: 2
Last year: 1
Last year: 3
Last year: 4
Last year: 5
Last year: 6
Last year: 7
Last year: 9
Last year: 13
Last year: 8