I'd imagine Chernarus is probably the most popular (I'm going off of Arma2 dayz experience). ALso, new custom maps like Tonoa and such tend to grow stale after the hype wears off with the community. At least that's generally how it worked in Arma2 dayz. Personally, I prefer Namalsk because it offers an actual survival experience to it. At least a harsher one. Making it Tonoa would probably limit the population of the server but I can't imagine it would be by much.
Chernarus = safe bet, tried and true
Namalsk = great for experienced players, not so much for newer players
Tonoa = biggest potential but risk cutting into pool of players available