What is the best rookie season by a pitcher in the last 100 years of baseball?

Has to be Grover (Pete) Alexander's rookie season in 1911.

He went 28 wins & 13 losses with a 2.57 ERA.

Fernando Valenzuela - Won both Rookie of the Year and Cy Young honors in 1981. Also started the 1981 All-Star game. Brought his team to the World Series.

From Wikipedia:

His efforts made him the odds-on favorite to be the league's top rookie in 1981, and he didn't disappoint, starting the season 8-0 with four shutouts and an ERA under 1.00. He became an instant media icon, drawing a huge crowd from the Los Angeles Hispanic community every time he was scheduled to pitch, and triggered a mad race to acquire his rookie baseball cards. After a strike wiped out a third of the season, he cooled down a bit, but still finished 13-7 with a 2.48 ERA and led the league with 180 strikeouts. He ended the season as the only pitcher to win Rookie of the Year and Cy Young Award honors in the same year. He pitched a complete game in Game 3 of the 1981 World Series against the New York Yankees

Fernando Valenzuela, not only the Rookie of the Year, he also won the Cy Young, started for the NL in the All-Star game and led the Dodgers to a World Series win over the Yankees, all in 1981.

He also had the city of LA in the grip of Fernandomania in 1981. You couldn't go anywhere in SoCal without hearing his name. He brought an entire culture to the ballpark. I dare say he brought millions of new fans out to Chavez Ravine

Dwight Gooden went 17-9 in his rookie year in 1984.

Bob Grim went 20-6 in 1954. Recordwise, I don't think anyone comes close since 1950.

1976 - Mark Fidrych (AL Rookie of the Year)

You could say Small from the Yanks, last year!!!! 10-0

Yankee Ron Guidry