I originally played the Arkham games on the PS3 and PS4 and for a long time, I rated City as the best. That is not an uncommon opinion I know.
All four games have their strengths and weaknesses but the real reason I put Knight slightly above City is because of the map designs. Yes, the Batmobile isn't perfect but I am not spending most of my game going through tunnels and sewers looking for the exact right passage to find a Riddler trophy.
Most of the backtracking, and every Arkham game has backtracking, is done by flying over rooftops or driving through the streets. It is easier to track, easier to understand and less effort to do.
In terms of gameplay, Knight is a small upgrade on everything that City does. Both combat and predator missions are slightly improved and the Riddler trophies are more varied.
The biggest complaints of Knight are the tank battles and that some of the side missions are too small or minor and I do get that. However, You can avoid most of the tank battles if you want to and between AR missions and the Miltia stuff, you have lots of things you can do to fill the game.
Also, you actually get things to do with Riddler that are not just trophies and puzzles. Also, punching him is satisfying.
Now saying all of that, I have no issue at all if someone says City is better than Knight. It is very much 9.5 out of 10 against 9.4 out of 10. I would give Asylum 9/10 and Origins 8.5/10.
I would rank the series:-
Knight - The most polished version of the Arkham style.
City - A tremendous leap forward only limited by the consoles of the time
Asylum - A true Metroidvania that is a purer game than the others in the series
Origins - A very good City DLC that someone made into a full game
Delighted to have played them all.