Of the ones I've read, I like Snoke being The Operator/Gallius Rax, an admiral from the old Emprie who cunningly rebuilt the left over forces into the First Order, which is sort of a nod to the EU's Thrawn. Much better than going with another Sith Lord that for whatever reason no one knew about.
Honorable mention goes to the theory of Snoke being Tarkin. Don't but into it, buy I kinda like it.