We-e-e-e-ll - and this may (or may not?) alienate some folks on my Flist, but I'll say it anyway - the people who I think are really overrated are the ones who sing and dance to songs that other people have written and played and produced. That's not real music, it's karaoke.
But then, effectively, they're not musicians.
So, looking up the first person that springs to mind, and checking whether he actually counts as a musician by that critera - and he does - Michael Jackson. He was okay, I suppose, a good entertainer, but none of his songs really grabbed me at an emotional level, and I could never really understand the level of his success.
Still, to each their own.