I have to disagree with the assessment being made in here that people are being stubborn or flat-out wrong by not acknowledging that MJ is number one. I believe that album sales and concert attendance are a pretty big part of determining things, but they are not the only factors. 50 Cent can sell more albums today than Stevie Wonder, but they day I admit 50 Cent is a better artist than Stevie is the day I stop listening to music. I like Prince and Stevie quite a bit because they play their instruments live and they sing a concert from beginning to end live, but I like MJ because of his energy and the scope of his concerts. However, the live music appeals slightly more to me.
I have Stevie Wonder and Michael Jackson as number one and number two, respectively. I think that when it comes to art, there is a certain threshold an artist can cross and when they do (people like Lennon, Dylan, Stevie Wonder, Michael Jackson, Frank Sinatra), the lines of "who is the best" begin to be blurred and there is no truly right or wrong answer.
There are LOTS of good arguements to be made why MJ is the best, but none of them are definite.