Probably the win over Kos.
It’s hard to compare because they were so different though. Kos had been a rising star at 170 for a while and had some notable wins but had also proven himself to be somewhat inconsistent. On paper he was a nightmare for most in the division; a d1 champion with great physical skills who had big power in his right hand. In practice though he sometimes found himself in bad spots where he probably shouldn’t have, and showed a propensity to both get cocky and be gunshy at different times.
Gilbert was also a great prospect on paper but had only had 4 fights in the UFC at welterweight and despite being remembered fondly as an action guy his only finish at 170 in the UFC came against the corpse of the then 43 year old Demian Maia.
Overall though I have to give it to Kos. After their first fight Josh talked about how he just wasn’t ready for GSP to try and wrestle him and had been overconfident, vowing that the Canadian would see a different man across the octagon from him when they met for the second time. Despite GSPs anointment as the next 170 leviathan it was hard to ignore the challenges that wrestler as seasoned as Kos posed for a man who relied so heavily on his ground game.