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.
