101 acting credits on his IMDB. About 30 of those were early career TV appearances and some movie roles pre or adjacent to Bonnie & Clyde, which was his first big movie. So let's say he made about 70 movies from that point on.
I've seen 40 of those (the man liked to work!). And I swear to god I can only think of one time when I thought he gave a bad performance. He was great as the star (The French Connection, The Conversation, Night Moves, The Royal Tenenbaums). He was great as a supporting player (Unforgiven, The Firm). He could elevate mediocre paycheck flicks (Uncommon Valor, The Package). He was a great smart antagonist (Crimson Tide). He was a great dumb antagonist (No Way Out). Known as a real pro's pro. Deadly serious on sets yet he could be as funny as anyone when called upon (Get Shorty, Young Frankenstein).
His best performances are the best performances everyone thinks of (many of which accounted for above). I want to go to bat for an underrated performance though. I rewatched The Firm in the last year or so. John Grisham ain't exactly known for his character development, but Hackman imbues his character with such a sense of sadness and regret. It's really incredible work in an otherwise frivolous beach read of a movie.
The one bad performance? Power, an 1980s political fixer flick where he gets acted off the screen by Richard Gere of all people. The one time I ever felt like he was phoning it in.