I'd thank him for everything he has given to and done for the world. I'd hug him if I wasn't too nervous. Then I'd probably ask him to teach me some singing and dancing tips.
I am not a fan of Michael, I am not a fan of his voice, that's all. I don't deny his impact and legacy though. So in all honesty, if I would have been able to meet him, I would like to talk about music. How the landscape was according to him in the time he was releasing. Also I would aks him what inspired him for his music and what his dreams and stuff would be if he never was in the showbusiness/music industry. What he thinks of the current music etc.
Probably get a picture with him, hug him and ask him how he is, etc - and then eventually go onto personal issues talking about the press, etc and ask him how he feels about the image people see him as. I wasn't really a fan of him, but from interviews, etc - he seemed like a great person.