I disagree with those who say that this question is equivalent to asking people to say whose music they dislike the most. I don't dislike the music of David Bowie but I could never really see what all the fuss was about with him. He was just another reasonably-talented (but not particularly special) artist to me. i.e. I regard him as 'over-rated'.
Similarly, while I can quite enjoy some of Ed Sheeran's stuff, I still seem his as being somewhat 'over-rated' (especially in out local press, where they never stop banging on about him because he's a local lad).
I do tend to agree with the suggestion of Kanye West above though. I admit to knowing little about him but his Glastonbury performance certainly didn't leave me thinking that I'd just watched an all-time great. (Don't get me started on Twenty One Pilots either!)
My own choice for the most over-rated music star ever would be Frank Sinatra. I can hear nothing in his songs that I can't hear in an average pub singer. For me, Bing Crosby was a vastly better singer (as were others, such as Dean Martin, Vaughn Monroe and Johnny Mercer)