5. Dizzy Dean (1934)
Dizzy Dean pitched for the Cardinals from 1930 until 1937. He was then traded to the Cubs, where his stats are much less admirable. During his time with the Cardinals, he pitched in 273 games, 1,731 innings, sported a 2.99 ERA, a 3.22 FIP, and threw 141 complete games.
During the 1934 season, he won 30 games (has never been replicated), threw 311.2 innings (interestingly enough, not his career high), had a 2.66 ERA, pitched 24 complete games, threw 7 shutouts, sprinkled in 7 saves, and had 195 strikeouts. While strikeouts are supreme in today's game, there is still a lot of respect for innings totals. To be able to throw 24 complete games AND have 7 saves is quite impressive.
Dizzy Dean's 1934 season was one in which he won an MVP. He could have also won the prestigious award in 1935 and 1936, seasons in which he was as dominant and reliable, but he instead finished in second place in both years. Dizzy Dean was second in WAR for all players behind only Lou Gehrig, but he racked up the most WAR of any other pitchers in the majors. His ERA, win-loss %, strikeouts per nine, complete games, shutouts, strikeouts, and FIP were all top-10 in the majors. Even for his time, he had one of the best seasons a pitcher could have.