The list, as published in Newsweek in May 1968, goes like this.
1. Nelson Rockefeller: longtime friends, and in LBJ's view, the only Bobby-beater in both parties.
2. Hubert Humphrey: loyalist, but a weak crybaby "he cries too much" and not capable of beating RFK. "I can't beat the Kennedys."
3. Eugene McCarthy: For what he isn't more than what he is. Rather inoffensive and LBJ always had a cordial relationship with Gene.
4. Richard Nixon: a sleazy, double-dealing, plastic opportunist.
5. Bobby: No explanation required.
So it's really a toss-up here. LBJ despises both of them, but never doubted RFK's political acumen or presidential ability. I'd say, if forced to choose, he'd go with Nixon.