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.
I doubt that Trafficante had the pull to whack Giancana. The orders definitely came from Accardo and Aiuppa, and possibly higher up. It's telling how Giancana and Roselli were involved with the CIA and both ended up dead in 1975 and 1976
