I had a hard time making a selection here because offense spiked so much during this decade. Juicing the ball in 1930 with Australian wool probably produced at least five or six legitimate candidates that year alone. In the end, I copped out a little bit and just went with the overwhelming numbers of Lewis Wilson.
1930 Hack Wilson (7.4 bWAR)
The barrel-chested slugger;s career had been trending up since he became a full time player in 1926. His home run, walks, SLG, and OPS increased each year since, very similar to how David Ortiz steadily improved from 2003 through 2006. After leading the league in long balls three straight years, he seemed to peak in 1929 when he teamed up with Rogers Hornsby to lead the Cubbies to the World Series. He set career highs in virtually every category and led the majors in RBI. But just when it looked like Wilson couldn't get much better, he took his game to historic heights in 1930, his age 30 season. It turned out that '29 was just a dress rehearsal for one of the best seasons of all time.
It was a perfect storm for Wilson to break Lou Gehrig's RBI record of 175, a number Larrupin' Lou reached three years prior while hitting in the heart of New York's Murderer's Row. Wilson batted cleanup in a lineup that finished first in the NL in home runs, walks, OBP, SLG, OPS, and total bases. The offense triple slashed .309/.378/.481, and the two team's number two and three hitters--Woody English and Kiki Cuyler--were perfect table setters. They OB'ed .430 and .428, respectively, and racked up tons of extra-base hits (but not home runs, so they were always in scoring position and didn't clear the bases). Not surprisingly, English scored 152 runs and Cuyler crossed home 155 times. Wilson started slow out of the gate, but a 22 game hit streak in late April/early May kickstarted his monster season. By midseason he was on pace for 164 RBI, only five more than his total the previous year, but he picked up the pace during the second half. He drove in 118 runs over the season's last three months and finished the year on a tear, knocking in 18 runs over his final ten games with a September hot streak that would make Troy Tulowitzki jealous.
His 56 dingers and 191 RBI gave him two-thirds of the major league Triple Crown. He also paced the Senior Circuit with his 105 walks, .723 SLG, 1.177 OPS and 177 OPS+. He also scored 146 runs, piled up 208 hits, and accumulated 423 total bases. Although no MVP was awarded that season, Wilson probably would have won it. As it were, the BBWAA recognized him as the National League's most "useful" player. His 191 RBI is one of baseball's most unbreakable records. Lou Gehrig (184--the American League record) and Hank Greenberg (183) challanged it during the decade, but in the 70 years since nobody has seriously threatened the mark. Manny Ramirez drove in 165 runs in 1999 while missing 15 games, but extrapolating his numbers to a 162 game season still only takes him to 181. Unless he went on a grand slam binge, I don't think ManRam would have eclipsed Wilson's record anyways.
Unfortunately Wilson lived, drank, and fought harder than he played. He reported to spring training 20 pounds overweight the following year, and predictably his production fell off a cliff Adam Dunn-style. He made matters worse by blaming Hornsby, the new skipper, for not letting him be as aggressive at the plate as former manager Joe McCarthy had. He found himself riding the pine in May, and by late summer Cubs owner William Wrigley Jr. expressed his desire to trade him. In September Wilson was suspended without pay for sparring with reporters while boarding a train in Cincy. He finished with his worst numbers since his days with the Giants, and his off-field issues left a sour taste in the organization's mouth. Chicago dealt him to the Dodgers after his OPS plunged nearly 400 points, and although Wilson did bounce back in 1932 he was never the same player. His swift decline forced him out of baseball before his 35th birthday. Although he only put together half a dozen good years, he did enough during that abbreviated stretch to earn a spot in Cooperstown from the Veteran's Committee in 1979, more than three decades after his death.
Some more big stats from Wilson's big year
-Laid down an astonishing 18 sacrifice hits that year, though that isn't too surprising when you see that he averaged 19 per year from 1927 through 1930
-His 56 home runs stood as a National League record until 1998, when Mark McGwire and Sammy Sosa obliterated it with their epic home run chase
-In 1999 the Commissioner of Baseball officially increased Wilson's 1930 RBI total to 191 after a box score analysis by baseball historian Jerome Holtzman revealed that Charlie Grimm had been mistakenly credited with an RBI actually driven home by Wilson.
-Drove in 109 runs during the season's second half. In 2010 Robinson Cano needed the whole season to plate 109 runners. In August alone, Wilson knocked in 53 runs.
-Loved hitting at Wrigley, where his OPS was 240 points higher than it was everywhere else that year. His 33 home runs and 116 RBI there match Ryan Howard's full season production from last year.
-Wilson led the league in strikeouts for the fourth straight year with 84 whiffs. Nowadays, that would be a fantastic number for a 50 home run hitter.
-The only team that gave him some trouble was the St. Louis Cardinals, who beat out the Cubbies for the pennant by a mere two games