tl;dr it's fucking subjective
I'll say taiko because of course, i play it and i like it. I don't play the two others modes.
Well i agree that CtB isn't a rhythm game at all, to me, it's just basically catching fruits and moving -nearly- randomly. But i respect people who play it, it's their choice. And to me, it's the easiest one to begin with.
Taiko is the hardest to begin with, but the easiest one to progress, once you know how to read patterns. Can't speak about mania, tho.
And Taiko perfectly makes sense in this game, the notes fits the rhythm and you mostly have to tap, unlike std where you hold some notes (sliders). but nowadays it's 1/8 sliders that counts as a circle \:D/