In the book "What Zoos Can Do" the purely subjective rating system of the author Anthony Sheridan highlights these 4 as Europe's best river hippo exhibits (in alphabetical order): Berlin, Cologne, Copenhagen and Valencia.
The exhibit in Berlin is much better, but both exhibits have one significant weak point: the very very small terrestrial part inside. The hippos are forced to spend the whole day in the water and the night in small boxes.
I've never seen an hippo outside here in Cologne. But the thing with hippo's is, is that they mostly feed at night and stay in the water at day time, because there skin is very sensitive. But the indoor enclosures of Cologne and Berlin consist of water and only a very small piece of land and I don't know if they are aloud to go outside at night (and even when they have access to outside it will only be in spring and summer, because of Europes climate).
The enclosures mentioned by Sheridan (Berlin, Cologne, Valencia) are all of the immersive kind. (I haven't seen Copenhagens one). But I think that the Antwerpen exhibit is much better suited for hippo's than the Berlin and Valencia one and possibly also better then the Cologne one, but I do'nt know about their back-stage facility's...
