Two were brilliant songwriters, one improved dramatically over the years, one was hopeless. So it can't be that.
The Beatles played very well together, but by no stretch could you call them "brilliant musicians".
Entertainers? I don't know about the band before they hit the big time, but I always thought their stage banter, and performance flow were very amateurish in Beatlemania years. I still think Paul and Ringo are stilted and self-conscious on stage.
So what were they, first and foremost? I think they will be seen, in hindsight, as a group, to have been brilliant innovators.