Re: a peak before a slide
Mosley and Roy Jones Jr. are two that spring straight to mind.
Jones looked edit: UNtouchable versus Clinton Woods, a very solid fighter (who went on to be a pretty good champ) and then moved up and white washed John Ruiz... We all know the story from there.
Re: a peak before a slide
Normally a fighter will have a peak performance and have a gradual decline in their performance such as Julio Ceaser Chavez or Terry Norris rather than a dramatic fall like Tyson.
A last hurrah would be Larry Holmes beating Ray Mercer.
Re: a peak before a slide
You never know what a fighters peak is until after the slide and decline. Julias Francis had a revival when he beat an undefeated Danny Williams, Pele Reid and another decent fighter that he was expected to lose. After he got demolished by Tyson he went back to normal losing most of his fights. Rogan beat Skelton and Audley was his peak.