As The All Music Guide says in their excellent Beatles biography "That it's difficult to summarize their career without restating cliches that have already been digested by tens of millions of rock fans, to start with the obvious,they were the greatest and most influential act of the rock era and introduced more innovations into popular music than any other rock band of the 20th century."
"Moreover they were among the few artists of *any* discipline that were simultaneously the best at what they did *and* the most popular at what they did." They also say as singers John Lennon and Paul McCartney were among the best and most expressive in rock.
Also on an excellent site,The Evolution of Rock Bass Playing McCartney Style by Dennnis Alstrand,Stanley Clarke,Sting,Will Lee,Billy Sheehan,George Martin and John Lennon are quoted saying what a great,melodic and influential bass player Paul has always been.
And Wilco's John Stirratt was asked in Bass Player which bass players have had the most impact on his playing and the first thing he said was, Paul McCartney is one of the greatest bass players of all time,if you listen to what he was tracking live in the studio it's unbelievable." "With his tone and musicality he was a huge influence,he covered all of his harmonic responsibilities really well but his baselines were absolutely melodic and inventive."
In this 2010 interview the blogger says that John Stirratt has an affinity for good melodies so it's not surprising that Paul McCartney is one of his musical icons and then he quotes him saying that he's always absolutely in awe of his playing,including Paul's Beatles years.
And in an online 1977 Eric Clapton interview,Eric Clapton In His Own Words he says that there was always this game between John and George,and he said partly because John was a pretty good guitar player himself http://www.superseventies.com/ssericclapton.html
He played live with John as a member of John's 1969 Plastic Ono Band.
And there is a great online article by musician and song writer Peter Cross,The Beatles Are The Most Creative Band Of All Time and he says that many musicians besides him recognize Paul as one of the best bass guitar players ever.He too says that John and Paul are the greatest song composers and that to say that John and Paul are among 2 of the greatest singers in rock and roll is to state the obvious,and that John,Paul and George were all excellent guitarists and that George is underrated by people not educated about music but that Eric Clapton knew better,he also says that both John and Paul played great leads as well as innovative rhythm tracks.
John Lennon co-wrote,sang and played guitar on one of David Bowie's first hits Fame in 1975 and David invited John to play guitar on his version of John's beautiful Beatles song Across The Universe.Brain May,Ozzy Osbourne,and Liam Gallagher and many more call The Beatles The Greatest Band Ever.'
Also on MusicRadar Tom Petty,Joe Perry and Richie Sambora in What The Beatles Mean To Me all say how cool and great they thought The Beatles were when they first saw them on The Ed Sullivan Show in February 1964 when they were just teen boys,Richie was only 5.Tom Petty said he thought they were really really great.
Robin Zander of Cheap Trick said he's probably one of the biggest Beatles fans on the planet.Brad Whitford of Aerosmith said that a lot of that Beatles influence comes from Steven Tyler's collaboration with Mark Hudson both whom are absolute Beatles freaks and he said I guess the goal is to try and emulate probably some of the best music of the last 50 years which has to be The Beatles.