Marichal, by far, especially for peak value. Oh was he feared. With his high kick, he was fun to watch and photograph.
Gaylord Perry was fascinating, because he had outstanding years with three teams as diverse as they come---the Giants, the Indians, and the Padres. With tongue in cheek, Perry was feared, too. It's tough to hit a spit-baller, and who wants to get hit with his slobber, when you're tryin' to clobber!
Growing up in the 60s, Marichal was always thought of as better than Perry or Neikro. Gibson started to get a lot of respect after the '64 Series, and he just kept getting better and better, but then angry Gibby ain't in the discussion.
If you want underrated pitchers, I'll chime in with Mel Stottlemyre and Dean Chance.
Look, I'm not trying to de-rail this fine thread. Keep it goin, guys.
---Brian Powell