Visually, the best I know is at Hamburg, Germany. It has an openable roof. Kudos for the climbing structures of various thickness, many high resting places, visual barriers and ropes allowing apes to swing. Wild orangutans use lots of swinging or swaying branches during moving. Unfortunately, once an orangutan drowned in the water moat. The living tree protected by hot wire is apparently the latest of succession of several expensive trees which either died or orangutans got to them.
However, like most orangutan exhibits, it has only one space for the group of apes. Orangutans are solitary in the wild, and specialists may think they value other things more.
The biggest exhibit I know is in Amneville, France
Again, big diversity of climbing thickness and above ground resting places, but no visual barriers letting the apes hide from each other, and visually rather bland. I think glass barriers are the best, because they allow apes to have lots of loose branches and manipulable objects, without fear of escaping on them like on ladders, or throwing at visitors.
BTW, grass usually survives in orangutan exhibits, and I think low bamboo or low scrub, or maybe also tall tree bamboo might work, too. Taller trees are, unfortunately, broken during attempts to build sleeping nests every night.