Big cats are the first ones that spring to mind for me, less so tigers, snow leopards and clouded leopards, for whom the enclosure standard in zoos seems to be, with of course a few exceptions, rather high, but much more so for leopards and jaguars, with lions seeming not to have it all too good, either. I have seen lions at over 20 zoos, and I think that on only four occasions was I truly astonished by the enclosure, those being Longleat, Woburn, London and Burgers', with the former two both having the advantage of being safari parks, and therefore having the ability to just enclosure sprawling fields without worrying about viewing angles. For jaguars, there is the unbelievably good enclosure at Chester, but other than that I can't think of any that stood out to me, and I can think of a few that are shamelessly subpar, and I don't think I have ever seen a particularly good leopard enclosure (although I have seen a fair few decent ones, such as Colchester or Marwell, with the latter now holding cloudies). What is perhaps strangest is that the worst enclosures I have seen for these species tend to be at major, wealthy zoos. The leopards and jaguars at Beauval, and the lions at Hagenbeck and Prague, for example.
Another one is my favourite animal - the hornbill. And although not to the same extent, most large birds, such as parrots and even birds of prey, can probably make the same complaint. Huge and majestic animals that, at times, can hardly stretch their wings in the cramped, dilapidated pheasantries they have access to at zoos. That said, there are certainly some welcome exceptions here - even more so than the big cats. Chester has multiple, in the elephant house, Monsoon Forest and Tropical Realm, while Antwerp has some smaller hornbill species free-flight in the Buffalo Savannah, but the best by far is of course Burgers' Bush which has free-flying Wrinkled Hornbills. One of my all time best zoo experiences is surely when said hornbill swooped mere inches above my head.
My final example would be mole-rats. On only one instance (the Mechow's Mole-rat enclosure in Africa Up Close at Prague) do I recall ever seeing them given access to deep-packed soil, or for that matter any natural substrate, in which they can form their own burrows. It is usually those nasty glass tunnels where they cannot dig, but seem desperate to do so and can often be observed scratching mindlessly at their glass walls. Truly heartbraking, and one of the few tropes of modern zoo exhibitry which I truly disapprove of - it feels like something out of a science-fiction movie, or a testing lab, not a serious zoological, conservation-oriented institution, and is making animal rights activists' jobs far too easy. Kudos to Prague for bucking the trend.
(Nothing to do with the thread topic, but this is my 1,000th post!)