To be honest, I am a little confused about the use of "peak" in this instance--seems like people are interpreting it differently from one another as well based on some responses in this thread. It could be seen as people who had untapped potential and fell off (Matt Allen, Scott Kane, etc.), or skaters who seemingly haven't peaked yet despite having been pro for decades (Guy, Daewon, etc.)
I'm going to think of it as skaters who were great, and seemingly had a future ahead of them, but for one reason or another (sponsor issues, partying, injuries, etc.) never "made it big"/got their deserved recognition/filmed that next video part we all expected and wanted.
Gailea Momolu is who always comes to mind. I fucking LOVED his part in Darkstar - Batallion. He was so damn smooth, great trick selection, and doing some really, really gnarly tech stuff down handrails. He seemed like he had a very bright future ahead of him. No clue what happened to him, but it's a damn shame that he only had this and like, a Logic part or something.