I don't think Kos was really a top contender. He got destroyed by Paulo Thiago and Alves during his title run.
It's hard to say.
The best WW he beat were Shields, Condit, and Hendricks.
Shields was an easy matchup for GSP, stylistically.
Condit was dangerous for moments, but we knew GSP would win the rounds.
Hendricks was pretty complete at that time, and that fight was very close.
I think the Condit fight was his finest performance.
I think I was most impressed with how he survived and kept a cool head after getting floored by the switch kick. Condit moved in to try to finish, but GSP (while hurt) kept his composure, moved his hips, and got out of there.
(For a lack of this champ heart, look at how Weidman dealt with being taken down by Rockhold and had none of what GSP had)