I think having Michael kill Fabrizio would be the most appropriate. It would rare for a don to actually carry out a hit himself because of the legal risks, but Fabrizio's betrayal hurt Michael more than anyone's. It could be argued he was one of the single most important characters in the trilogy. If he hadn't killed Appolonia with that bomb, she would have come back to America with Michael as his wife and, more than likely, lived a life very similiar to Momma Corleone. Michael would have had a family much more similiar to Vito, being loved and revered, as opposed to despised and resented, by those around him. For those reasons, he would have personally wanted to kill Fabrizio himself.

If the scenario Joe Batters presented had been filmed, I think it would have been a nice personal touch for Fabrizio's assasin to be Calo. He continued to work with the family through Don Tamassino. Calo uttering that, "Hey kid you look familiar. Fabrizio, regards from Corleone" line, then seeing the "oh shit" look on Fabrizio's face as he figures it out would be priceless.