Georges St-Pierre has been in some wars. Nick Diaz. Johny Hendricks. Matt Serra.
None of those guys was his toughest opponent, however.
Who was it? Carlos Condit and BJ Penn, according an interview GSP did with bloodyelbow.com
Penn and Condit. Condit hurt me the most, and then it was mentally a challenge, and Penn also hurt me. People think Hendricks was the toughest, but against Hendricks I was emotionally hurt, that's what showed on my face. The next day physically I was fine. Against Hendricks I felt I was against the system. I felt the system was corrupted against me, like Dana White was for some reason against me, and I didn't understand why; I was not trying to put the UFC down, but to bring ‘em up. When I spoke before the fight I wanted to help the sport, I wanted to help the UFC, so that's what showed, what was emotionally, mentally in my head. I felt the system was against me and I didn't understand why.
GSP retired after his split decision victory over Johny Hendricks in 2013. The fight was a battle and many felt that Hendricks won the fight. But clearly GSP said it was Condit and Penn who did the most damage.
GSP has hinted that he may come back soon. He recently agreed to be tested by the United States Anti-Doping Agency, a sign that he is thinking of returning to the Octagon. Many dream fights exist with GSP back in the mix, including GSP vs. Conor McGregor, GSP vs. Nate Diaz and GSP vs. Anderson Silva.
St-Pierre has said publicly that he would fight anyone if he returns to the UFC. St-Pierre has defeated every fighter he ever faced and with a record of 25-2 was widely considered the greatest MMA fighter in the world before his retirement.