Which incarcerated felon gets the most fan mail?

I don't know that murdered Scott Peterson gets any more mail than any other fenon but he certainly gets his fair share:

San Quentin inmate Scott Peterson has been receiving as many as 25 pieces of mail a day in the month since his arrival on Death Row, the majority of the messages from "mainstream America" and supportive, says San Quentin spokesman Vernell Crittendon. "Much of that mail is from people who do not know Scott Peterson personally and most of them are females ... Three of them I recall were actually wedding proposals to him." Crittendon tells the Associated Press, adding: "There have also been a number of media requests for interviews."

https://people.com/celebrity/female-fan-mail-for-inmate-scott-peterson/

Manson remains imprisoned to this day (2008). More than nine applications for parole have been denied, most notably in 1986 when he appeared before the parole board with a swastika embossed on his forehead. During his stay in prison, Manson has received more mail than any other prisoner in the United States prison system. It is said that he gets over 60,000 pieces of mail a year; much of it is fan mail from young people hoping to join The Family.

http://www.religioustolerance.org/dc_charl.htm