This is a tough decision! I saw the polar bear exhibits at Rotterdam, Ouwehands, Dierenrijk, Monde Sauvage and Emmen last summer. I also visited Pairi Daiza but the polar bear exhibit was under construction at the time and not yet open, plus AZ Friesland is a place I didn't visit and there is also a brand-new polar bear exhibit there. I might wait and see what zoo nerds say about those exhibits, as both look to have certain high qualities.
Monde Sauvage has an extraordinary polar bear exhibit because half of it is an old-fashioned cement bunker with very high walls that that looks like this:
The newer section of the polar bear exhibit at Monde Sauvage is a forested environment that seems more appropriate for brown bears than the great white bear of the tundra:
Monde Sauvage is decent, but it would not get my vote.
I would also not vote for Emmen, as the viewing areas are a bit tight for the number of visitors and there are big metal pipeline things like this that are aesthetically unappealing:
Ouwehands has a pool that is 100% green in the summer and thus there really isn't any underwater
viewing although the exhibit is a good one with some thick grass and many boulders to create a lush-looking environment. I can see why Ouwehands could have some supporters.
Rotterdam has absolutely superb underwater viewing, with huge windows that are spectacular. The two outdoor exhibits were probably world-class when they opened almost 20 years ago, but now are simply okay. This zoo might get a few votes as well.
Dierenrijk would probably get my vote, although I'm going to wait to see what people say about the new exhibits at Pairi Daiza and AZ Friesland. Here are 6 photos that I took of the two enormous polar bear exhibits at Dierenrijk. The lack of underwater viewing is a negative, but there are many positives with the two spacious enclosures at this zoo.
A polar bear cub at Dierenrijk:
A polar bear about to dive into the water: