This list has changed quite a bit since I first posted it 2,5 years ago, so I decided to redo it (with a bunch of extra categories). All of them are species/zoos/exhibits I've seen, of course. I added a bit of explanation to the categories that are most important to me.
Mammal species (captive): Cat Ba langur, Checkered elephant-shrew, Chinese pangolin
In my previous edition of this topic I also had a langur, elephant shrew and pangolin but I feel like I've now "upgraded" all of those to a subjectively more special species. There's a lot of honourable mentions though, and this list would probably change every other day.
Mammal species (wild): Black-shanked douc, Delacour's langur, Azuero mantled howler
Not sure why, but all of these ended up being primates. I'd say I like Black-shanked and Red-shanked douc about equally, but my observation of Black-shanked douc was much better so that's what made the list. I also really wanted to include Brown-throated three-toed sloth, Moose and Burmese ferret-badger, but thinking back about it all three of these monkeys got me a lot more excited so deserve to be on the list more.
Bird species (captive): Long-wattled umbrellabird, Greater bird-of-paradise, Sickle-billed vanga
I'm sure it was predictable that this list would mainly be made up of cotingas and birds-of-paradise... I limited myself to one each, though, to keep things a bit more diverse. The third slot had to go to a species I never could have dreamed of seeing, so it was between the Sickle-billed vanga and the Cuckoo-roller, both at Walsrode (which coincidentally makes this list Walsrode-only).
Bird species (wild): Resplendent quetzal, Bar-bellied pitta, Eurasian wryneck
I don't have a favourite bird, but if I would it'd be Eurasian wryneck, which is so unique and gets bonus points for it being a species I can see in the wild in Belgium (with a bit of luck!). The other two, I felt, had to be the representatives for my two most birdy holidays to date - Resplendent quetzal was the easy winner for Panama, and while Vietnam was harder to decide for I think it has to be Bar-bellied pitta.
Reptile species (captive): Bornean earless monitor, Domergue’s leaf chameleon, Olive-brown sea snake
Reptile species (wild): Hawksbill sea turtle, Siamese crocodile, Banded krait
Amphibian species (captive): Three-toed amphiuma, Olm, Hellbender
Amphibian species (wild): Alpine newt, Sa Pa spadefoot toad, Rainforest rocket frog
Fish species (captive): Manta ray sp., Black-spotted electric ray, Leafy sea dragon
Fish species (wild): Smooth hammerhead, Moray eel sp., Guineafowl pufferfish
Invertebrate species: Trilobite larvae sp. (wild), Vagrant emperor (wild), Chambered nautilus (captive)
Zoos: Zoo Antwerpen, Burgers' Zoo, Weltvogelpark Walsrode
I had originally split this off in Major and Minor zoos, but it felt weird to have 3 minor zoos in this list that I liked less than unlisted major zoos. I think Antwerp will likely be on here for the rest of my life, but perhaps the other two slots will switch - Bronx, Singapore, Night Safari, Jurong, Berlin ZG, Leipzig, Prague and even wildcards like Zlin were all serious contenders for this spot as well.
Exhibits: Burgers' Bush (Burgers' zoo), World of Birds (Bronx Zoo), Open Ocean (S.E.A. Aquarium)
S.E.A.'s Aquarium might be a surprise to some, given that I generally don't pay too much attention to fish, but there's something absolutely magical and humbling about standing in front of a gigantic slice of the ocean and seeing three gigantic manta rays elegantly break the vastness of the water. The Bush is the best representative I could think of for my favourite kind of zoo exhibit (large tropical halls), and World of Birds is the best representative I could think of for my second favourite kind of zoo exhibit (large bird houses).
Guilty pleasures: Red-billed leiothrix, Violet turaco, Bali myna
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Like most other people in this thread, all of my opinions change very often so you'd likely get another response tomorrow.
