In 1884, Charlie "Old Hoss" Radbourn started 73 of his team's 112 games. In these games, Old Hoss went 59-12, setting an all-time record for pitching victories.
He also led the league with a 1.38 ERA and 207 ERA+. He was as effective as Sandy Koufax in his prime, pitching 2-3 times as many innings.
By the end of the season, Old Hoss couldn't raise his arm over his head. He couldn't dress himself every morning. That responsibility fell to his roommate, and manager, Frank Bancroft. Pitching through excruciating pain, he led the Providence Grays to a National League tittle.
Radbourn's 1884 season takes the cake as the greatest season any pitcher has ever had. But the game was different back then. Since 1900, baseball's modern era, no pitcher has approached Radbourn's 59 wins.
Even so, the game has seen it's share of dominant pitching seasons, from a 20-year-old phenom who took baseball by storm in the 1980s, to a pair of 34-year-old 300-game winners proving they were far from finished.
No pitcher will appear on this list more than once. A few could have, but I limited it to one season per pitcher.