RE: this topic, Rod Stewart is the first name that came to mind for me. His work with Jeff Beck was great. The albums he recorded with Faces are some of the best rock records of all time. And his first three solo albums are amongst the very best albums of the 1970's, or any era for that matter. However, his work, post-1977, has varied from middling to downright awful.
I think Greil Marcus hit the nail on the head when he said of Stewart, "Never has there been a more naturally-gifted rock singer than Rod Stewart and never has a singer betrayed his gift more completely."