So, to take up this challenge...
I have visited 351 European Zoos. A large number of these can be discarded straight away, either because they're now closed down (Prague Terrarium, Windsor Safari Park...), really tiny (Zoo am Meer, Bremerhaven; Bentley Wildfowl Park), or really awful (Pecs Zoo, Genk) - with some falling into more than one of those categories.
There are also a number of places I've not seen, which would have a good shout for inclusion in such a list - I've not been to Lisbon, to anywhere in Greece or Italy, to Berne or Gdansk, to any Baltic or Scandinavian zoos (apart from Copenhagen). In the former Soviet Union, I've only been to two zoos. In France, I've not yet made it to the Parc des Felins.
So, from the places I have been to, here are the 73 I would definitely include:
- Tiergarten Schonbrunn Austria
- Antwerp Zoo Belgium
- Paira Diaza Belgium
- Planckendael Zoo Belgium
- Jersey Zoo Channel Islands
- Brno Zoo Czech Republic
- Dvur Kralove Zoo Czech Republic
- Jihlava Zoo Czech Republic
- Ostrava Zoo Czech Republic
- Plzen Zoo Czech Republic
- Prague Zoo Czech Republic
- Zlin Lesna Zoo Czech Republic
- Zoo Ohrada (Hluboka) Czech Republic
- Copenhagen Zoo Denmark
- Bristol Zoo England
- Chester Zoo England
- Cotswolds Wildlife Park England
- Dudley Zoo England
- Hamerton Zoo England
- London Zoo England
- Longleat Safari Park England
- Slimbridge Wildfowl Trust England
- Whipsnade Zoo England
- Amneville Zoo France
- Jardin des Plantes, Paris France
- Mulhouse Zoo France
- Parc de Cleres France
- Valle des Singes, Romagne France
- Zoo Beauval, St Aignan France
- Zoo de Vincennes, Paris France
- Zoo Doue la Fontaine France
- Allwetter Zoo, Munster Germany
- Augsburg Zoo Germany
- Berlin Tierpark Germany
- Berlin Zoo Germany
- Cologne Zoo Germany
- Dortmund Zoo Germany
- Dresden Zoo Germany
- Duisburg Zoo Germany
- Frankfurt Zoo Germany
- Hagenbecks Zoo, Hamburg Germany
- Hannover Zoo Germany
- Heidelberg Zoo Germany
- Krefeld Zoo Germany
- Leipzig Zoo Germany
- Magdeburg Zoo Germany
- Munich Zoo Germany
- NaturZoo, Rheine Germany
- Nuremburg Zoo Germany
- Rostock Zoo Germany
- Walsrode Vogelpark Germany
- Wilhelma, Stuttgart Zoo Germany
- Wuppertal Zoo Germany
- Budapest Zoo Hungary
- Dublin Zoo Ireland
- Fota Wildlife Park Ireland
- Apenheul, Apeldoorn Netherlands
- Artis Zoo, Amsterdam Netherlands
- Blijdorp Zoo, Rotterdam Netherlands
- Burgers Zoo, Arnhem Netherlands
- GaiaZoo, Kerkrade Netherlands
- Opole Zoo Poland
- Warsaw Zoo Poland
- Wroclaw Zoo Poland
- Moscow Zoo Russia
- Barcelona Zoo Spain
- Carbarceno Natural Park Spain
- Fuengirola Zoo Spain
- Madrid Zoo Spain
- Valencia Aquarium Spain
- Valencia Zoo Spain
- Basle Zoo Switzerland
- Zurich Zoo Switzerland
In response to the discussion, above, about the inclusion of 'mainstream' collections that aren't of the world class quality of a Berlin, Prague or Zurich.... I think
@FunkyGibbon is being a little unfair to, as an example, Heidelberg Zoo. Heidelberg is not an 'international' zoo. Apart form Nerds Like Us it's not going to attract people coming to the city specifically to see it, in the manner of a San Diego, Chester or Paira Diaza, nor does it have the quirky specialism of some other mentioned zoos, but it is, nonetheless, an excellent establishment, a really good zoo, with a number of really good exhibits, and I would eb really delighted to be able to visit it, again and again. Opole, in Poland, is another such place - nothing earth-shattering nothing exceptional, just a really good zoo that it is an absolute pleasure to visit. I'd give my right arm to have such a place - or Wuppertal or Krefeld or Brno - as a 'local' zoo.