20. Clayton Kershaw
If Clayton Kershaw were to retire today, he would already be a Hall of Famer. Kershaw is one of the most dominant regular season pitchers in MLB history, leading the league in ERA five times before the age of 30. Kershaw has also won an MVP, a Triple Crown, a Gold Glove and three Cy Young Awards. Once he retires, Kershaw will go down as one of the most decorated pitchers in MLB history.