Ive just read this thread and I got the feeling that most of the posters on this thread aren't really in a position to offer an opinion, not an informed opinion anyway.
To compare the solo careers of the four I think you would have to judge them between 1970 and 1975.Lennon released several albums in the early 70s, most notably Imagine and Walls and Bridges, Imagine is the one Lennon album that is considered a stand out album. Lennon stopped recording in the mid 70s and didn't release a new album until Double fantasy ( which was a poor effort ) in 1980. Harrison only made one stand out solo album "All Things Must Pass" which came out in the early 70s, a fantastic album comprised of tunes Harrison had written during The Beatles era, its probably kinder if we don't mention Ringo. McCartney, like Lennon, released several albums during the early 70s, three of these albums are stand out albums, McCartney, Ram and Band on the Run.
It depends on how much knowledge you have of their work after the Beatles, some people have been influenced by the music media and others have listened to all the albums and have made a more informed decision.
In my opinion McCartney was almost certainly the best all round musician in The Beatles, he definitely had the strongest singing voice in The Beatles, and he was probably the strongest songwriter in The Beatles. I think the work that the four of them put out between The Beatles officially splitting up in 1970 and Lennon stopping recording in the mid 70s proves this statement to be true.