I'll toss in yet another vote for Sal, who was absolutely NOT on anybody's radar [especially after getting decked in that draw with Juan Escobar at the Olympic in his American debut]. Danny was the biggest lighter weight star champion the States had in the professional ranks during the late 1970s. His classic 1979 Ring FOTY against Mike Ayala had experts speaking in whispers. At the absolute pinnacle of his career, he was a cover boy on Sports Illustrated, and got the full color glossy centerfold of the inaugural issue of KO Magazine after stopping Jose Caba. [A match in which he looked just fine.] Nobody dreamed he'd already posted his final career win. Everybody was anticipating a Lopez-Gomez superfight. KO's inaugural issue profile said if Danny won that one, he'd dominate the FW division as long as he cared to. Within the ranks at 126, it seemed there was nobody left. Sanchez-Lopez I was projected beforehand by CBS to be a match which might produce a lot of knockdowns. Little Red was obviously the reason viewers tuned in. [Such an upset of Pedroza by Sal wouldn't have made Sanchez the name in the English speaking market which beating Lopez turned him into.]
James Toney when he beat Michael Nunn. His only notable win was a SD over Sosa and everyone thought Nunn was on his way to becoming an ATG MW before Toney knocked him out. A couple years later and James is considered the P4P #1 boxer in the sport.
Pipino Cuevas was a kid, an 18 year old teenager, with a so-so 15-6 record and coming off of a loss against Andy Price when he destroyed the respected Angel Espada in 2 rounds to win welterweight title. He then went on to rule the division for several years with his bone breaking power.
Are you asking about fighters no one had ever heard of making a splash or fighters who were unheralded and then burst through to the top level? I had heard of Kelvin Seabrooks before he all of a sudden caught fire and won/defended a world title, but no one would have bet a wooden nickel that he would've jumped to World Class the way he did.
Casamayor and Spadafora both came up around the same time. They both beat established fighters to win titles without having a serious win on their resumes or being ranked by The Ring.