When I say that the Doors were capable with their instruments, I really mean it, they certainly knew very well how to play them. But I don't know what you mean when you use the word "virtuoso"; to me, virtuoso is Eric Clapton, Jeff Beck, Jimmy Page, not to mention Jimi Hendrix (could Robby Krieger have competed with them?). I enjoy Ray Manzarek and he was a basic ingredient in the Doors' sound, but he usually reminds me to an amateur circus keyboardist; I prefer Rod Argent or Steve Winwood much more. And I like John Densmore but he wasn't much more than an average drummer.
I prefer to consider the talent of an artist based on their own merits and not by comparison to others.
About Manzarek playing bass lines with the keyboard, I understand that it was only in their debut album; on their next releases they used a session bassist.
I was specifically referring to their live club performances.
But hey, I did put the Doors among the greatest 60's bands, so I certainly respect them very much.
This is why I don't particularly care for lists. They don't tend to be very objective.
But you did preface your list by saying "Of course, the unavoidable subjective factor is still there."