Undoubtedly Fifa 15 is the best version. Once you start playing it, you will definitely feel the difference. The graphic is too good compared to Fifa 14 ( and earlier versions) . Gameplay is quite different, more fast and smooth. Besides the customization is much more flexibly. For example, you can set a players position manually with navigation keys. Moreover, the passing and through balls are a bit difficult and obviously more realistic than Fifa 14.
I have also played Fifa 16 which doesn’t stand a chance in front of this one.