First time jumping in to rate zoos I haven't been to. Given the quality information and great discussion though I'll take a leap and look at Plzn and Pairi Daiza, particularly because I'd actually love to visit both.
First impression I agree with Kalaw and the very 'flash' Pairi Daiza seems an easy win. But there are lots of things to like about the early view of Plzn, not least their large collection of mongoose which are some of my favourite animals (though dwarfed literally and figuratively by the larger carnivores in both collections and I get not everyone loves 234 variants on a meerkat). I'd go to a collection that just had all the mongoose.
I would also say that if we debated theming in regard to Land of the Lions in the Berlin / Whipsnade thread Pairi Daiza takes the cake, makes a model of it, whacks on 20 candles and pops a tiger in it.
Thinking about the overall cup and zoos in general I find I am more swayed by exhibitry than variety / collection size animals, but the qualification of a great exhibit for me is around husbandry and welfare as well as spectacle and context. So I'll be looking for carnivores to be housed in great enclosures with interesting signage and opportunities to engage with information and of course hang about to watch the animals.
I did take a wander around the zoo official websites
Discover Pairi Daiza
If the zoo is an event hub, this highly professional website is the window into 'experience' lead curation. It's a little telling perhaps that the main homepage links are orientated to the visitor and the animals are a layer below, contained in their experience centres. I guess this reflects the actual collection where the animals are in these regions.
Still, some wonderful photos and information on the collection. But a bit more focus on the human areas than the other animal ones.
Leads me to ask people who have visited - is it hard to find your way around PD or is the navigation and signposting sufficient to identify the animals you want to see? I do factor this into a rating as in order to appreciate a thing you have to be able to find it.
Plzn has a few links but I think this is the official site.
Zoo Plzeň
It's obviously a lower budget affair, but it has a lot of information on the areas and animals and nice features on the gardens.It's easy to find information on the animals (grouped by geographic region) and other activities. I felt I learned something about the zoo from the site (carnivores and wider) vs finding my way to booking an experience so, as such, a more conventional 'zoo' site than Pairi Daiza.
Moving on to the zoos themselves, Some wonderful animals on both lists. The smaller carnivores look particularly attractive for Plzn but it can come down to how they are shown. PD clearly takes the seals, Sealion and Polar bear space. I'll revist when I have looked at exhibits and read a bit more.
I'd like a bit more information on the carnivore over stocking issues at PD referenced here as I am not familiar with the discussions on them and it sounds like a material piece of information. Are there particular species that are over stocked, is it from over breeding or bad design and is it a temporary, historical, recent or simply persistent issue?