There are now 7 lists on this thread, which is a useful number to reveal a certain level of consistency with the choices being presented. I would encourage others to take a stab of coming up with their own possibilities for a Top 100 as it's a fun thing to do and 'the more the merrier'.
A trio of things that I've been pondering:
1- I think that for a travel writer, a book about Europe's Best Zoos would need to have a certain degree of geographical representation. That would open the door for lesser-known zoos to make the cut based on the fact that they are the #1 major zoo in a particular nation. Examples of this would be:
Croatia - Zagreb Zoo
Estonia - Tallinn Zoo
Finland - Helsinki Zoo
Greece - Attica Zoo
Latvia - Riga Zoo
Norway - Kristiansand Zoo
Slovakia – Either Bojnice Zoo or Bratislava Zoo
All 8 of those zoos, in 7 different nations, have popped up at least once each on this thread, but whether any of those establishments are truly amidst the very best zoos in Europe is a question that I'm ill-suited to answer as I've yet to visit them. In terms of geographical representation they'd be strong contenders, but in all honesty the 20th best German zoo could perhaps be a stronger candidate than any of them. I think that a book written for zoo nerds might not contain a single one of those 8 zoos, or only have one or two selections at most. If someone wants to write about the absolute best, then geographical positioning needs to be tossed out the window. If there are 25 zoos from Germany and 0 from Latvia then that's just how things roll.
For instance, in the list provided by
@sooty mangabey , he had 64% of his choices from only 4 countries!
Germany, France, England and
Czechia accounted for 47 out of the 73 zoos listed. He didn't have space for any zoo from Croatia, Estonia, Finland, Greece, Latvia, Norway or Slovakia.
2- What could be viewed as an automatic choice might not in fact deserve to make the cut. Not everyone would approve of Madrid (some outdated enclosures), Barcelona (currently in-flux) or Moscow (hardly visited) to make a Top 100 list. 'Name recognition' isn't enough for zoo nerds.
3- I have a gut feeling that the Scandinavian countries are being under-valued due to lack of visits from zoo enthusiasts. I could be way off-base, but
Denmark has Copenhagen Zoo, Blue Planet Aquarium, Aalborg, Odense, Givskud and Randers.
Sweden has Kolmarden, Parken, Nordens Ark and Boras...all interesting zoos. I'm not sure how many would make a Top 100 list, but those two nations have impressive animal welfare laws and from those that have visited it would appear that both Sweden and Denmark have a handful of top-class zoos.