@Lain
Let's not act like Arkham Asylum and City's villain weren't lame as well. Aka steroid Joker and Ra's Al Boring (would have been way better if Strange was the actual main villain the whole time... talk about stupid sublopts and twists).
In the face of that, and the fact that AK has the most refined gameplay of the four games, I think it definitely is the best game in the series. I also don't care what anybody says, the Batmobile would be a complaint of mine, if it wasn't fun as all heck.
Arkham Knights was an ok game, it was good, but "refined" is a bit of a stretch. Arkham City is the best in the series. The game design as a whole was refined from Asylum. It kept you wanting to play more. In Arkham Knight the driving was cool, but it took - you, the player - away from the whole Batman experience, including the fights.
However, I think the batmobile was put in not just because WB said "put it in," but I have a feeling that players (their customers) wanted vehicle driving. Thing is though, it could've been like an option, rather than making it forced upon you. It seems like every mission is designed around the batmobile. So much, they forgot what made Arkham City so legendary.
Oh, and Joker in both Asylum and City was sublime. Whatchoo talkin' about!? Especially City... Joker was like the deepest evil villain in there... and his minions was a great experience to battle against. Thing is, though, I agree they could've switched it up a bit. Which they did in the second half of City, and into Knight.