Hi MW,
I would suggest the firmest that was “comfortably sleepable” for people that needed or preferred a firmer surface (such as stomach sleepers or those that were a little heavier than some of the others).
Then I would add a couple of thinner toppers in the 2" - 3" range (softer latex or memory foam) or a fiberbed (which would just provide some surface softness for those that wanted it) to the “menu”. I would stick with 2’ for latex and the memory foam (if there are some who like memory foam in the group) could be a little thicker say 3" which would give a bit more of a memory foam feel.
So not hard as a rock no 
… but also not so soft that adding toppers would compromise alignment.
There are a couple like this that have say 2" of memory foam on one side and then 2" of polyfoam on the other or other combinations (such as the innomax memory foam/latex topper here). You can also find quite a few mattresses that have side to side split layering for a couple with different needs and preferences or use say two twin XL of diferent types on a King mattress. there are also mattresses that have zip covers where the inner layers can be removed and replaced with a different layer (The Dreamfoam has their Eurotop for example that has a zip cover where the top layer can be exchanged and other manufacturers such as those listed in post #21 here have mattresses with zip covers where any of the layers can be exchanged or replaced).
Phoenix