Who Held The Single Season Record For Isolated Power Relative To The League Average Before Babe Ruth Came Along?

George Hall. He did it in 1876. Below are the top 10 seasons from 1876-1918. Hall's ISO divided by the league average is 3.14 (.179/.057). Times 100 is 314. This comes from the Lee Sinins database and there may be some rounding issues here. Ruth broke the record with 353 in 1919. Hall's 316 is still 9th all-time. Only Ruth and Gehrig ever did better (Ruth has the highest with 433 in 1920).

Player	YEAR	REL ISO	ISO	LG ISO
George Hall	1876	316	0.179	0.057
Fred Dunlap	1884	292	0.209	0.072
Ross Barnes	1876	285	0.161	0.057
Ned Williamson	1884	283	0.276	0.097
Buck Freeman	1899	280	0.245	0.087
Dan Brouthers	1881	269	0.222	0.083
Honus Wagner	1908	265	0.188	0.071
Lip Pike	1876	263	0.149	0.057
Tip O'Neill	1887	261	0.255	0.098
Tom York	1878	260	0.156	0.060

Hall's team scored 7.61 runs per game at home in 1876 and only 4.76 on the road. So his park might have helped him. But they allowed 9.09 at home and 9.00 on the road. Baseball Reference gives him an OPS+ of 204 that year, 3rd highest in the NL from 1876-99 after Barnes (235, 1876) and Brouthers (208, 1886).

Hall's last year was 1877 at age 28. His OPS+ fell but was still a good 132. Wikipedia reports he was banned from baseball for betting. The Baseball Library has similar information.