I never really paid attention to this thread as it seemed to be "just another" personal top ranking. Personally I have visited 18 of the 50 institutions and admittedly would not consider many of them to be on my list of personal favourites. After reading the explanation intro I understood the different approach. Not having seen the majority of these zoological institutions I cannot really evaluate everything. Yet under the given criteria I am definitely missing Hannover Zoo here. I know it's a zoo that is hated by many traditional zoo fans, yet since this list is explicitly not about "better/worse" or personal liking but about zoo styles and setting standards you cannot go around Hannover. It's maybe THE style for 21st century zoos, copied already by Gelsenkirchen and Emmen - and in parts while not to the full text extent by several other zoos.