The first one that comes to my mind is "The First Cut is the Deepest". Sheryl Crow murdered that song, yet it became a giant hit. Too bad Yusef Islam doesn't give a shit anymore, because he probably had a lot left to record. Also "Jersey Girl". Love Waits. Hate the boss.
I absolutely loathe her version of that song. Cat Stevens was super influential on me during some formative years and to hear her hack through it... cuts the deepest. That song placed her fully into the same category as Rod Stewart - No Talent Ass-Clowns who have no originality, and a voice like sandpaper - and not in a good way. It's sad - because I used to kind of even like Sheryl. She had a few songs on earlier albums where the rasp in her voice worked in her favor - but she's dead to me now. And though I will concede that she probably had a "bigger hit" than Cat - it's garbage to the core. It's proof that even a great song can be ruined with ease.
I have been listening to a lot of Cat Stevens of late. My daughter really likes his music, I do too. You're right, that guy likely had a lot left to create especially if he kept stealing Joe Satriani riffs