The original wasn't Ubisoft though, it was CryTek, so it had an entirely different formula and feel. It wasn't truly open world, it was story-driven, time of day was geographically locked for the sake of the storytelling and ambiance, etc. That's what made it special and unique to the series.

When Ubisoft took over for the second game, the selling point was "there arr no f***ing mutants in this game". And from there it was just usual Ubisoft cookie-cutter shameless cut-and-paste, where every zone had the same gun vendor NPC. You could leave his shop, jump into a car, drive like hell across the map, run into another shop and...there he is! They didn't even bother to put a unique NPC vendor in gun shops, that's how copy-pasty the game was.

Though I gotta give Ubi some points for going no-HUD. There was no HUD, no minimap, no nothing. The game was crazy immersive and could have been ported amazingly well into VR today. You actually had to hold a map and a compass and figure where you are, and you couldn't do this and shoot at the same time.

Far Cry 3 was decent, and actually had a fun skill tree system with tattoos. But I feel Far Cry 4 is where the series peaked. The variety of activities and NPC interactions in the world, the story, the "hidden ending", the missions themselves. The usual hallucination thing that Ubi loves so much was still there (reaching Shangri La), but you could also choose which faction to side with (out of the 3, technically, if you include Min). It was pretty damn unique for the series.

And I wouldn't say FC6 was the worst in the series, that one goes to New Dawn. The health bar nonsense in that one was x1000 compared to FC6, they tuned it down considerably in FC6. And they also borrowed a little too much from other games, as usual, such as anti-air missile batteries from Ghost Recon, which made the game less enjoyable and more grindy than it could have been. And the amigos instead of a pair of companions who could banter back and forth with each other, not just the player, like in FC5, was a major step back in FC6. But they added cutscenes with player character in third person as a tradeoff, if the writing wasn't so atrocious it might have been worth it.