Some of the best water I've seen in any game period. I went down to the water's edge and just took a bunch of pictures. It's incredible.
What really gets me though is the scale to density ratio of the world. Infamous was pretty, and it was big, but it felt really empty and static. Arkham Knight has so much destructible junk everywhere, driving the Batmobile feels like I'm playing Red Faction, minus the large-scale building destruction. And every NPC seems extremely detailed.
All this with barely noticeable pop-in (usually just grass) and super fast traversal in a large city, solid 30 framerate (save when crashing into a ton of physics objects in the Batmobile, but that only lasts for two seconds at best), and full HD.
Honestly, it's hard to be that impressed with other games after playing this. I'm sure there are titles that can beat it in individual categories, but I don't think there's an open world that runs this well and looks this good.
Not to mention the animation...
The only thing that tends to look a little wonky is the hair, but its still a step above a lot of other games.