I'm sure there are cases where a fighter took on a future champ in their early stages that would rank ahead, but as far as already established 'best' Prado fighting Phil Davis has to be close to it.
Others have pointed it out, but it's Yuji Nagata, by far. http://www.sherdog.com/fighter/Yuji-Nagata-3422 Prime Fedor and Prime CC in your first and only 2 fights. It doesn't get tougher than that.
UFC debut or simply pro debut? In terms of UFC debut, Joe Lauzon faced and defeated Jen Pulver, who was seen by some as the current UFC Lightweight Champion.
Stephan Bonnar vs. Brian Ebersole (both made their pro mma debut together). Forrest Griffin debuted against Dan Severn.
that special mention to kenneth williams http://www.sherdog.com/fighter/Kenneth-Williams-2370 0-2 with his 2 opponents being rampage and chuck liddell, neither rampage or chuck were in their prime but still worth a mention.
That's cool. I just looked at that fight card and saw Miguel Torres fought and won two times that night.