Its Football Money League, based on season 2009-10, also said the combined revenues of the 20 clubs had passed 4bn euros (£3.8bn) for the first time.
Real's arch-rivals Barcelona retained second spot in the list, ahead of Manchester United who remain third.
Manchester City were the biggest climbers, up from 20th to 11th place.
Arsenal, Chelsea and Liverpool were fifth, sixth and eighth respectively.
Corporate partners
Seven of the top 20 in Deloitte's table were from England, the other three being Manchester City (11th), Tottenham Hotspur (12th), and Aston Villa (20th).
All the 20 clubs represented are from the "big five" European leagues, with Germany contributing four clubs, Italy four, Spain three and France two.
In a further sign of a financial hierarchy at the top of the European game, the same 10 clubs populated the top 10 places in the Money League for the second successive year.
And top six ranking places are identical to last year. Meanwhile, six of the top 10 have been in the money league top 10 in each of the last 10 years.
The authors said that Real may now be about to emulate the eight years spent by Manchester United at the top of the rich list, from the first edition in 1996-97, through to 2003-04.
They also said that they had expected to see the full impact of the global economic downturn on clubs during the 2009-10 season.
"We continued to assert that the game's top clubs would be well placed to meet these challenges given their large and loyal supporter bases, ability to drive broadcast audiences and continuing attraction to corporate partners," the report says.
"This was more than borne out by clubs' revenue performance in 2009-10."
Deloitte also says that the established large and loyal supporter bases and historic on-pitch success has "continued to underpin the brand strength" of football's top clubs.
Source
