Hi
@lala2021, I also like to go to zoos when I travel and Chester Zoo is definitely on my bucket list. Really hope I'll be able to visit someday.
Being from Belgium, I can recommend you Pairi Daiza. It was voted the best zoo in Europe several times. It's absolutely stunning. But be warned, it's so huge that it's almost impossible to see everything in one day. (There are hotels in the park though with views on some animals. Amazing but quite expensive).
I also love Antwerp Zoo very much. I have had a membership there for several years. It's very different from Pairi Daiza. Smaller zoo right in the middle of the city but still very interesting. And very beautiful in its own way with lots of historical buildings. It's one of the oldest zoos in Europe with a lot of historical value, like London or Berlin. And they have Okapis.
If you could visit only one, I would still recommend Pairi Daiza first because it's such a crazy amazing place. But Antwerp is also very much worth it.
The Paris zoo was completely renovated a few years ago and is interesting but less than Antwerp or Pairi Daiza IMO. They have manatees though.
There is also a very big and famous zoo in France called ZooParc de Beauval, which seems really great. I've never been there but I've been thinking about it for years and I should normally go this spring or summer.
Speaking of Denver Zoo, I've lived in Denver for a few months back in 2012 and went there a few times. I liked it. But I know they've changed a lot of things since then and I don't know if it's still good or not.
The Smithsonian National Zoo in Washington, D.C. is also worth a visit and has the advantage to be free.