Interesting question, I’ve been mulling over this one! My opinion is Blues, Rhythm and Blues, early Rock n Roll are the easiest.
Blues- standardised chord progression with variations, three chords, I V IV progression, often only pentatonic/ blues scales, also use of static riffs- riffs repeated over changing chords, without changing key.
The simplicity of this music led to mass audience through radio and juke boxes, paving the way for rock and roll, rockabilly, and popular music in general.
Btw: I’ve got a King Curtis CD in the car that’s in the player and it’s been played over and over for weeks. Good driving music, I recommend!
I guess it seems that the genre is based on preference. Most people I know who done lessons started with classical…and for me the combination of learning to do sheet reading and classical…well, was overwhelming.