Why debatable, make your case!
In fact when it comes to the vocal harmonies, there is no question about it, as the Everlys were bigger in Britain than they were in the US, and there was a duo in the UK at the time of the early Beatles that sounded just like the Everlys, called the Allisons (there was also another one whose name I forget at the moment.) That’s my case.
Everlys were an influence for sure.
But the Beatles rocked more, check out their Hamburg recordings.
John Lennon himself stated nothing affected him til Elvis.
Other obvious influences are Chuck Berry, Little Richard, Motown (McCartney said Motown was the biggest influence on his bass playing).
The were influenced by Buddy Holly and even were inspired to use Beatles as their name as a play on the Crickets.
They covered a Holly song, Chuck Berry, Berry Gordy, other Motown.
Not sure they ever covered an Everlys song.
I say they had so many influences none can really be said to be the biggest.