Let me take a minute to explain why I think so. Apart from being extraordinary performing musicians, outstanding songwriters and fascinating personalities, the Beatles invented an entirely new way of composing. This took place from Rubber Soul on. Stepping away from their exhausting touring commitments, they spent over a month in the studio writing, arranging and recording the all new songs. Compare this to the mere ten hours they spent in the studio for their first album. What they were doing was composing for the studio. In other words, they were creating unique soundscapes for every song using available resources and inventing new ones as needed. Some examples of the latter include "varispeed" recording where you alter slightly the speed of the tape recorder. When you play it back at normal speed, it alters the quality of sound. They used this technique to make the instrumental tracks sound "fatter" and the vocals sound more forward. They also used double-tracking on the vocals. A solo vocal line was recorded twice. Since it would never come out exactly the same, there was a added presence to the line. As this was very time consuming, one of the technicians invented a way to do this automatically. It is called "automatic double-tracking" of course and now it is available as a software effect. They did a lot of other things too, like recording voice or guitar lines and playing them back backwards. Or using tape loops to create a kind of "musique concrète".
So from Rubber Soul on, a Beatles song is, in its recorded incarnation, a whole aesthetic object. Inevitably, most cover versions take the bare bones of the song and render them in the style of the artist. Here are a couple of examples. the first is a pretty good version of "A Day in the Life" by José Feliciano:
And the original:
Of course, the reason the Feliciano version is pretty good is that he goes all-out to try and reproduce the original, even to the extent of having a full orchestra backing him up!
Here is an arrangement/recomposition of "Penny Lane" for guitar and orchestra by Leo Brouwer:
Not too bad, but I would prefer the original as it is more uniquely what it is, than when it is turned into a guitar concerto:
But, as I was saying, this is not a foregone conclusion and there is a new album that might be rather interesting. I ran across this courtesy of Norman Lebrecht at Sinfini Music. If you are a really good string quartet with a really creative arranger, then the Beatles offer some interesting challenges. Here is a link to the Amazon page where you can listen to brief samples of the songs on the album. There seem to be no clips on YouTube of the Wihan Quartet playing the Beatles, so have a listen to the brief samples and let me know what you think.