Serra vs GSP 1, clearly.
Reasons being that GSP is a greater, more rounded, better fighter than Crocop,
the Serra loss came in the middle of a great run both before and after (coming off wins over Hughes, Penn, Sherk, Trigg, Miller, going on to wins over Koscheck, Fitch, Hughes, Penn, etc),
Serra was really a lightweight and GSP a big natural welterweight,
the fight only came about because of the TUF agreement, Serra was not and probably never would have been considered a legit #1 contender without the show,
Serra had lost his last fight before the show (to Karo Parisyan),
it was just a much bigger surprise, the level between the fighters was bigger. Randleman was coming off two losses, but they were to superb fighters (Sakuraba and Rampage). Those guys are miles ahead of the guys who'd beaten Serra in the UFC (including Shonie Carter and Din Thomas). Randleman was always perceived as dangerous, Serra was not.