Of the two words for "now", (kounye a e koulye a), which is the most used? Mesi.

That depends on what part of Haiti you're from.
Some people say kounye a
Some people say koulye a
Some people sa kounya
Some people say konnya
and sometimes we say kounye a la :)

Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words