The Isley Brothers would have to be on any list for me (though they also continued post-Beatles)
I'm not sure how the opening poster wants to define "pop" pre-Beatles. Could go all the way back to some of the great music hall and vaudeville performers, such as Dan Leno, Harry Champion, Marie Lloyd, etc. And cabaret singers such as Edith Piaf,
Is Glenn Miller Pop and Duke Ellington jazz? Do you only mean singers?
If you include producers/writers then I can't believe no one has listed Phil Spector on here yet, and by extension the groups he produced/work with. And songwriters such as Goffin/King and Holland/Dozier/Holland.
Sorry to be difficult. Just wondering what you mean exactly...
All Glory to the Hypnotoad!