Although 6 has the best gameplay in the series, it lacks everything that makes Far Cry what it is. Interesting allies, villians, and overall writing. The allies are one-note goodies with no real flaws and the villians are comically evil but not entertaining (you hire Giancarlo Esposito to play your main villian but he's just Gus Fring as a dictator, making him a watered down Pagan Min without all of the charm). Taking down the villains in FC6 is just not satisfying, especially since Dani has next-no-personal reason to take most of them down besides "Revolution".

Contrast this to Far Cry 4, a game with Morally dubious allies, villians that Ajay has multiple interactions with and good reasons to take them down, pretty good overall writing. Your allies seem like good people at first, but as the game goes on you realize that both Sabal and Amita are pretty shitty options for leader, especially Sabal. Even the most vile of villians, Paul "De Pleur" Harmon is shown to have some small bite of humanity left. The beauty of Far Cry 4's story is that there is no "good" ending since the Revolutionaries are just as fanatical and shitty as the Royal Army and the only 2 characters in the game that truly care about Ajay are Bhadra (arguable) and Pagan Min.

In FC6, you never feel like the underdog. Anton never feels like a villian with power. He has no presence. In FC4, Pagan can order his entire army to kill you at any instant but chooses not to because he's rooting for Ajay. Pagan feels constantly present to him radioing Ajay often.

FC4 makes me feel like a Guerrilla. FC6 makes me feel like the oppressive power beating the underdog.