Since It Happened One Night is one of the best films ever made so I tend to side with that as being Gable’s best performance. However, I always mix up the names of Clark Gable and Carey Grant_ and I have no idea why.
Red Dust with Jean Harlow. It’s amazing!
Edit: I forgot Call of the Wild! Good movie as well. And he and Lorretta Young had time to make a love child while making the movie.
But you should look that up on your own:)
Run Silent, Run Deep
It Happened One Night
San Francisco
Call of the Wild
Men in White
and, begrudgingly, GWTW. I really didn’t like the movie, but he is great in it