We all know Michael Jackson was one, Madonna is one, the beatles were, Diana Ross was one once, Elvis. But there are lots of stars who are/were big but are they classed as a superstar? Rod Stewart, Tina Turner etc. They have been big but would they qualify for superstar big?

For someone to be referred to as a supertar i think they need to have the X factor.
There has to be something about them that sets them apart from their rivals, they have to packaged well, they need to be controversal and really push the boundaries, and also set trends instead of following them. A superstar is someone who continuously re-invents themselves and stays one step ahead of the game.

A superstar must be creative, daring, and energetic.
They must captivate the world!!!

I think there are very few legendary, iconic stars, but i would say the following are:

The Beatles,
Elvis Presley,
Michael Jackson.

These are not just supertars, but they have legendary status.

I would say Superstars to name but a few are:

Madonna,
Celine Dion,
Mariah Carey,
Stevie Wonder,
Britney Spears, ETC......................

I can sort off see where you are coming from with Madonna and Stevie, but I certainly wouldn't describe Celine Dion, Mariah Carey and Britney Spears as acts who "push boundaries", "continuously re-invent themselves", or "stay one step ahead of the game". But it's all down to opinion I suppose.