The magazine values the Premier League champions at £1.19bn, which comes from an analysis of income from sponsorship, merchandise and ticket sales.
The valuation will be of interest to the Red Knights, the group of City financers looking to seize control of the club. The Knights are said to be in talks with a range of potential investors aimed at raising funds to launch a bid in excess of £1bn for the club.
The Red Knights and the Manchester United Supporters Trust are angry at the £700m debts current owners the Glazer family have saddled the club with.
Manchester United is one of the most attractive sports clubs in the world to sponsors because of its global fan base. United signed an £80m four-year deal with American insurer Aon last summer and has agreements with Nike, Hi Seoul, Hublot watches and Betfair. The team recently agreed a partnership with African mobile phone operator MTN Group, Telekom Malaysia and Turkish Airlines.
Real Madrid was rated second by Forbes on £859m, while Arsenal were third on £767m.
The Spanish team topped Deloitte’s Football Money League last month with earnings of £278.5m. Manchester United dropped from second to third place.