the only two I had in mind.
GSP was the obvious choice of going out while on top.
But we will NEVER know how he would fare had he stayed. Maybe he'd lose his very next fight, or maybe he wouldn't. but I am 100% sure he was still close to the top of his game, his prime.
And Khabib, unfortunately his father passing is what took him out, so it was not even that he was tired or declining..... A pity. There were lots of good fights for him that we'll never get to see. Then again, lucky Islam! He would probably not have ascended if Khabib was still at the top.