Paul might be the only one you could have made it without The Beatles.
That's an interesting thought.
1963 - Oppotunity Knocks - 2 contestants by chance from Liverpool, Paul MCCartney and John Lennon, each step up to the mike and sing a song, accompanied by their own guitar.
Who would have got through? Paul for sure, I guess. John wouldn't have had the looks/stage charisma to be a solo artist*. Songwriter yes. Sole frontman in a band without a McCartney? Again, he might have got away with it, but nothing too successful**.
Would have made a good Rolling Stone.
Paul solo - yes. Sole front man of a band - yes.
George, no and no.
Ringo - yes, as some kind of drum novelty act (The Amazing Ringo Starr and his Singing Drums!)
* F**k. Now I'm thinking Roy Orbison. I give up.
** hold that thought. If someone as personality free as Steve Winwood can front Traffic, then John could have fronted another band. (Sorry-stupid on my part)