Few names spring to mind, Don curry, George Foreman, Jeff Lacey. Prince Naz of course never recovered from his loss to Barrera.
Not quite what you're looking for because he wasn't undefeated. But Victor Hugo Paz of Argentina built up an impressive 56-1-1 record going into a fight for the vacant jr. lightweight title against a Korean, whom he lost to. After that he went 16-54-4 with a couple of no contests thrown in. How's that for downhill?
Fernando Vargas. He looked pretty bad in his first fight after Tito (Wilfredo Rivera). He looked somewhat better in his next fight, then lost in an excellent fight against DLH. The beatings he took in those losses and the back injuries really took a toll. Meldrick Taylor. He did have some good performances after losing to Chavez (Aaron Davis) but look at how fast he declined after that. Watch the Davis fight, then watch the Glenwood Brown fight (10 months later), then his fights against Terry Norris and Cristano Espana. It was such a rapid decline.
Elisha Obed. As soon as he quit against Dagge, he went from phenom to journeyman to bum in short order. Really a complete freefall.
If we don't want to count the hit Griffin when he was down disqualification then Roy Jones' quality dropped precipitously after his first loss to Antonio Tarver.
Cotto went 32-0 including wins over Mosley and Judah. He then went 9-6 including being dominated and KO'd by a substantially smaller fighter, schooled by Austin Trout and beaten by Saddam Ali. From a purely statistical standpoint, Teddy Yarosz went 58-0-1 then 48-18-2. Larry Holmes was clearly a much lesser fighter post his loses to Spinks than pre them, though that was age related.
John Mugabi was never the same after Hagler. Imagine a fighter that had the ability and toughness to go 11 hellish rounds against Marvin Hagler, giving almost as much as he got. Than watch him get ko'd in the 1st rd against Duane Thomas, and Terry Norris. He was clearly diminished after his war with Hagler.
Not his first loss, as you would expect for the era, but I would mention Terry McGovern. He is a good example of a fighter who one loss destroyed.
