Cy Young and Christy Mathewson, but neither one could match the sheer volume of Happy Jack's record-setting 1904 season.
Jack Chesbro 1904 (9.9 bWAR)
After winning 70 games in the previous three seasons combined, Chesbro added a spitball (which he learned from Elmer Stricklett, the inventor of the pitch) and "slow ball" (change-up?) to his arsenal and became even more devastating. He got on a roll early, winning 14 straight decisions from May 14th through the Fourth of July, setting a club record that lasted until Roger Clemens strung together 16 consecutive victories in 2001. His 239 strikeouts ranked second behind Rube Waddell's record-setting 349, but stood as a franchise record until Ron Guidry passed him in 1978. In almost any other year, his 1.82 ERA would have been good enough to lead the league, but he had to settle for fourth.
Chesbro's 51 starts, 48 complete games, 41 wins, and 454 and two-thirds innings pitched are still records for the modern era. Manager Clark Griffith deployed his ace so frequently that Chesbro made 28 starts with less than three days' rest. For good measure, Chesbro also topped the league in appearances (55), winning percentage (.774) and lowest hit rate (6.7 H/9).
Unfortunately for the Massachusetts native, he is remembered more for his infamous wild pitch on the final day of the season, a gaffe that allowed the winning run to score and cost his New York Highlanders their bid for an American League pennant. The mistake tormented the Hall of Famer for the rest of his life and nearly caused him to retire that winter, but he developed a new pitch called the "jump ball" and came back strong the following year, though he never again came close to replicating the magic of his 1904 season.
Best offensive season of the 1900s--Nap Lajoie (1901)