In September 1953 the 7th and 8th of 16 teams become integrated, if you try to squeeze in a “True HR King” between integration and expansion it would most likely be Ernie Banks. 10% more teams in 1961, 9% more teams in 1962, 20% more teams in 1969 and 8% more teams in 1977.
Mike Schmidt hits the most HR 1970–1986, 1971–1986…1985–1986. 1987 is a crazy season that almost definitely involves MLB tampering with one or more things to increase HR. There is no realistic explanation revolving around PEDs and/or ballparks that wouldn’t also impact 1986 and/or 1988. 1987 is not “True Baseball”.
After 1987 and before silly ball you can make a case for Fred McGriff as the “True HR King”. Cecil Fielder is another option if you give him at least partial credit for his 38 HR in Japan in 1989.
Sadaharu Oh is the “True HR Emperor” with 868 HR in a 6 team league 1959–1980.
With Alex Rodriguez never failing a PED test after the 2003 anonymous survey there is no credible basis for a “True HR King” in the last 25 or so years. “Never failed a test” is a completely meaningless standard. Of course Mike Schmidt and his contemporaries never even had PED tests to beat.
Anabolic steroids aren’t invented until 1958 but testosterone supplements in baseball go all the way back to Pud Galvin in 1889 using the Brown-Sequard elixir and various non-testosterone PEDs would also have been available to Ernie Banks.