sometimes significantly so. He was usually amongst the league leaders in the major pitching statistics, but rarely paced the AL in anything. In four World Series he went only 2-5, but could not have pitched much better as he completed six of his seven starts and had a cumulative 1.32 ERA. You could make a case for him, based on longevity, as the ace over Grove, but I don't think that is wise.
For me, the third guy is easily James Augustus "Catfish" Hunter (1965-74). After five mediocre seasons to start his career, he won 18 in 1970 and then 21 in each of the next three years. In 1972 he posted a 2.04 ERA, but 1974 was his best season with the A's as he went 25-12 with a 2.49 ERA (led league) and 23 complete games to take AL Cy Young Award honors. He was generally excellent in the postseason as well, going 7-2 with a 2.55 ERA.
Chief Bender (1903-14) was a quality arm alongside Plank during the early years, so he'll be the fourth starter on this roster. He won 20+ twice, and 17+ another five times. His best season was 1910 when he was an oustanding 23-5 with a 1.58 ERA (though again, the league average was 2.37, so his mark only ranked fifth best).
Along with Plank and Bender, lefty Rube Waddell (1902-07) pitched in the first decade of the team's existence. In 1905 he took home the pitching Triple Crown: 27-10, 1.48, 287 strikeouts. His ERA was consistently better than the league average for the day, largely the result of his ability to strike batters out: he led the AL in that category every year for the A's, and had a career high 349 in 383 innings in 1904.
After two brief seasons, Vida Blue (1969-77) burst onto the scene in 1971 as a 21-year-old phenom. He went 24-8 with a 1.82 ERA, striking out 301 in 312 innings (with 24 complete games), and took home both the Cy Young Award and the MVP Award in the American League. Although he never again pitched at that level, he did manage to win 20 games in two more seasons for the A's, and also had seasons of 18 and 17 victories before leaving for the Giants in 1978.
Dave Stewart (1986-92, 95) bounced around with several teams for several years before becoming an intimidating workhorse starter for the Athletics during the 1986 season. He won 20+ games in four consecutive seasons (1987-90) before losing his effectiveness. He didn't win the Cy Young in any of those seasons, though he finished in the top four in the vote in each. And he was consistently good in four postseasons for the A's, going 8-3 with a 2.31 ERA. And here is some nice trivia: on June 29, 1990, Stewart pitched a no-hitter against his future team, the Blue Jays, at The SkyDome. Only hours later, Dodger Fernando Valenzuela no-hit the St. Louis Cardinals at Dodger Stadium, marking the