Tanoa is a really great map but it sucks that you need to buy the dlc to be able to play it. Namalsk is a fun map to because of the massive environmental change of pace and its free so you can attract a larger variety of people.
But also do we really need another server if very few people play on the first
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
I would honestly prefer namalsk because like dots said its a harder survival experience plus its easier to find people with fewer people on the server because of the map size.