Which singers would you say are vocally mediocre but nonetheless great artists?

As the title suggests: which singers do you think are mediocre vocalists but strong artists?

I personally think Madonna is a great example for this. Her vocal prowess is often understandably criticized by many, to the point where people claim that she can't really sing. She's not necessarily bad but definitely underwhelming compared to her rivals.

Still, she managed to be one of most influential music stars, having against all odds a highly regarded discography under her belt, with many of her songs having been masterfully created with great production, etc. Heck, I even grew to like some of Madonna's ballads more than I do with, say, a lot of the vocal trinity's (songs that they're generally known for) despite the fact that her ballads are far less impressive vocally than theirs simply because of how well crafted some of them are.