At work yesterday we were talking about how big Michael Jackson was and who else is/was in the same league as him.   The thing is that Michael Jackson was no mere superstar, he was a Megastar and not many people reach Megastar stardom.  I see megastars as person who has changed the world with there singing, dancing, acting etc and who is know the world over and who can do they own work.

I would say who are megastars..  Elvis Presley, Madonna, Bob Marley

I wouldn't place Britney Spears or Arnold Schwarzenegger as a megastar myself even though they are both big stars.

Someone at work said that the Queen of England is bigger than Jackson lol. 

absolutely no one......if were just talking about music career

In music? Madonna, Bob Marley and Elvis are shoe-ins of course. The deaths of Jim Morrison and Jimi Hendrix roughly had the same effect as MJ's.
Frank Sinatra is/was a megastar.
When Eric Clapton dies, he'll be on the same pedestal.

Yeah that's all I can think of now

In Music.

Only Elvis Presley.

And, this sounds daft, but Cliff Richard possibly... his fanbase is epic.

Bruce Lee, Muhammed Ali, Micheal Jordan

BEATLES? Oprah? Bill Gates?

There are still plenty mega famous people.

John Lennon
David Beckham

add them to the list.

Tom Cruise. Hollywood could'nt afford him. He had to start UA again. He was apparently the first (still?) actor whose films grossed over 8billion collectively.

Elvis
Mario
Jesus
Chuck Norris

David Bowie. Should have been mentioned by now...