My favorite tends to jump between FFIX, FFXII, and FFX. It's really hard to pick one because they all do things that i really like. ideally the best FF would be a combination of these games.
As for FFXVI i don't have high expectations for it. So far I'm just not seeing what i'd like to see when it comes to Final Fantasy but maybe i'll change my mind when they show more of the game.