Clemenza with Rocco as a capo regime, and maybe also Neri if he was around at the time.
Looking back it was a mistake by Barzini not to make plans for Michael's "accidental" death because that would have backed Vito, who promised revenge, if anything happened to Michael, into a corner. Did he really have the muscle to take out "some of the people in this room?"
If Michael was killed by Barzini in Sicily, I think it would be a fair assumption to make that Vito would have went into full war mode and would have no doubt have called in all of the "favors" he had been sitting on all that time. With nothing to lose it would Vito would have probably be killed eventually, but not before he'd have wiped the Tatagglias out and maybe Barzini too.
If Michael had died along with Appolonia, there would never have been a meeting, so Vito would not have known that it was Barzini all along.
Vito may have gone into war mode and destroyed Tattaglia, but Barzini would have sat back and watched his cat's paw and his enemy weaken each other. It would have been ideal for him.
The other possibility was that Vito would have been totally destroyed by Michael's death. He was already slippin', personally weaker and at the head of a Family on the way down. He may have stepped aside and told Tom to negotiate a settlement, which would have led Sal and Pete to fight each other for control.
Or he would have allowed them to form their own Family, which would have been doomed to failure under an untenable co-Donship.
In any scenario, I think that, if the car bomb kills Michael, Barzini wins.
