So many variables! Do you mean from the point of view of a competitor, or of a social dancer, or of a complete novice?
Each dance requires steps and technique. Which are the easiest steps? Which the easiest technique?
The waltz, it can be argues, requires the fewest steps to be learned. But it needs a lot of technique.
The merengue does require Latin hip motion, so I doubt that a complete novice would find it the easiest. Same goes for chacha, mambo, rhumba, etc....
East Coast Swing requires you to learn a fair number of steps, but very little technique (at least at first), so I think I would suggest ECS for a complete novice.
My 2cents worth.
Jerry