Butch Blasi is the one mentioned most often. Others include Tony Spilotro and Chuckie English. My guess would be Blasi. Chuckie English remained loyal to Giancana and may have been killed for his loyalty.

Lots of strange goings-on relative to Giancana's murder. Some people maintain that Santo Trafficante had Giancana killed because of his knowledge of the Kennedy Assassination. Johnny Roselli was murdered about a year later for presumably the same reason.