I'll update mine, since I think I forgot some on my previous post and I'll add short comments on these.
UK:
-Chester Zoo: My favourite, contains a diverse collection with many fantastic exhibits in a beautiful setting.
-London Zoo: Fewer larger species, but it's London Zoo! What more needs to be said?

-Whipsnade Zoo: Fantastic collection, large enclosures and is London's baby brother!

-Jersey Zoo: Keeps many rare species and works very hard with in-situ projects.
-Twycross Zoo: The most varied primate collection in the UK, mixed with classic zoo animals and rarer species.
-Knowsely Safari Park: Good safari park featuring quite a few untypical safari park species (congo buffalo et al)
-West Midlands Safari Park: Similar to Knowsely (anoa, prezwalski horses) yet holds a few drawcards such as the white lions and tigers.
-Longleat Safari Park: National interest due to popular BBC tv program, fairly standard collection from memory (stand to be corrected though)
-Blue Planet Aquarium: Seemed impressive from my visit, haven't been for a while though.
-London Aquarium: Can't remember too much...
-National Sealife Centre: see above...
-Stapely Water Gardens: An interesting collection for a small animal attraction...
-Monkey Forest: Interesting to see the group dynamics of 2 troops of over 200 (?) barbary macaques
-Reaseheath College (it has a zoo license - that counts

): I have fond memories of this place...
US:
-Disney Animal Kingdom: Beautiful scenery and an interesting collection, better than I expected.
-Busch Gardens, Tampa Bay: Fantastic animal exhibits (from memory) especially the gorilla and chimp forest and the hippo enclosure.
-Sea World, Orlando: What you expect really...
Spain:
-Barcelona Zoo: Good to see Snowflake, but still a typical european city zoo...
-Terra Natura, Benidorm: Better than I expected, had a great S.American aviary and the park features a few rare species.
-Mundomar, Benidorm: Standard dolphin park, but features a good "zoo animal" collection
-Safari Park Vergel: "Standard" european safari park
-Marineland, Mallorca: Basic dolphin park.
-Auto-Safari, Mallorca: hopefully this is now closed, awful place!
-Green Planet Reptile House, Mallorca: now closed, but an impressive, well displayed reptile collection