Most likely Reagan, Rocky was seen as a RINO (though that precise term wasn't invented yet) by the base even in '68, Romney was a Muskie/Giuliani analogue in having a roof but no foundation, etc. Maybe Percy, but he's just elected in 1968 and will really be running for VP. If Kennedy is the incumbent the GOP will definitely want Reagan because only he can counter Kennedy's charisma and hold on WWC voters, but Reagan seemed adamant that he was going to finish his term IOTL. He refused Ford's offer of the vice presidency and multiple Cabinet positions in 1974 IOTL (plus the Veepship in '68), and given Reagan's stubborn devotion to principle I don't see him making an about-face in 1972.