16. They generally tend to improve, dissimilar to games like COD, where certain versions are revered and others lamented. They're all essentially the same in format, but they just improve on minor details, AI, and so forth.
You'll want the latest version for latest squads/player ratings, etc.
I'm looking forward to FIFA 17, as it's finally introducing a new game mode, rather than just Player/Manager career, or FUT/online.
FIFA 16 drafts is a cool addition, too. So yeah basically 16.
Eh I preferred 14. 16 has a couple issues. Dribbling is overpowered, pace above anything is overpowered and unless you play sweaty as fuck, you won't win online lol