I think this comes back to why I like Paul's solo stuff more so then the others. I think of Paul as I do the Stones in that most of their albums are a mismatch of stuff with some filler, but theres usually always that one or two songs that just floor you. Of course they had exceptions with solid albums all the way through (or most of the way), but hopefully you understand what I'm getting at.
Getting back on topic, more then a few times, my buddy and I have listened to 70's Elton John and I sat there and said, none of the Beatles solo stuff has ever been this good. I don't know if that's true or not, but that's how I feel. Could it be that I'm just so used to the lads solo stuff and can no longer give it a fair shake? Who knows.
In conclusion - I think Paul would have been a pretty big star in the 70's according to his solo output. He has enough variety and legendary songs that would push him into the forefront.
John, had a few legendary songs, but damn it if I don't feel like almost most of his solo stuff sounds all the same. He wrote Imagine so he would be remembered for that. The rest of his solo stuff? Not a lot there in my opinion.
I enjoy George Harrison solo records, but i'll be the first to admit that most of it is boring, twangy, and sounds a lot alike. He wrote enough stuff to be remembered, but I doubt he would be associated with anything huge or mind blowing. Throw him in the James Taylor category.
Ringo had a few hits, and a string of albums I thought were excellent, but I have a big gripe with Ringo and its that he always needs outside help to get a product made. Lets face it, he struggles with songwriting chores. Nothing to be ashamed of, but it happens. Hell, tons of musical stars out there never wrote a song in their lives. Ringo would merit some mention due to some air play with 'It Don't Come Easy' and 'Photograph', but it starts to slide pretty drastically after that. The 'No No Song' would have gotten some play too. I'd give him props for being somewhere in the hierarchy of a 10cc or Climax.