For me, it would be the Beatles, then Elvis and then Michael.
But I remember an old Bobcat Goldthwait routine where he asked if anyone ever got into one of those pointless arguments about who was the greatest rock star of all time. Then he said that it is inevitable that someone is going to say Elvis. He said he could understand someone who grew up in the 1950s feeling that way because they grew up in a time when Elvis really was the King of Rock and Roll. They saw him in their formative years.
So I think it mostly comes down to who someone is, when in their own lives and what time period it was that they became aware of someone. The era of the Beatles was what I grew up on although I already knew who Elvis was but his height was before my time. Nowadays, I find Elvis's range a bit limited. For instance, John Schneider did a rendition of one of Elvis's hits but put back the subtleties and nuances that Elvis just couldn't do.
Again, for me, it's the Beatles, then Elvis and then Michael. In fact, I discovered Michael because he did Thriller and the word was out that Vincent Price did a part in it that I wanted to hear. But, again, it comes down to age and my background more than any of them being objectively the best or the worst of the three.