I think it's the one that you like the music most. Me, it was Foxtrot and swing. But that's because I enjoy the big band sound and fifties music.
But as for Rumba being the easiest to follow, I think that's because of the speed of the music. It's slow enough that you can easily correct in mid-step the follow, and the lead, because of the slowness, can usually be the clearest. Once the lady gets the basic tempo that is. I find it's usually the mix of slow and quick/quick that someone people have a hard time picking up on.
-TheDitz