Were they still open, London would easily be the best for me, with Bristol a respectable second. It’s heartbreaking how within a span of just four years, the country lost both of its best aquariums.
My personal favourite is by far Whipsnade. Plenty of endangered species, linked to extensive conservation work and often very rare (at times almost non-existent) in captivity makes this house a treat for all fish enthusiasts. Similar things could be said about Chester’s aquarium, which is probably the largest in the UK by species count these days, with over 80 species on my visit in 2021, but the tanks there are repetitive, box-like and at times appear to be far too small. Whipsnade’s tanks are far more interesting, with some of the best landscaping that I have ever seen in an aquarium, even including independent attractions. There are floor-to-ceiling displays, but also paludariums, with geckos, chameleons and pythons roaming above the fish, which can be very entertaining. I don’t know of any UK zoo which does paludariums so well. Tanks are assorted geographically, themed to obscure aquatic ecosystems and almost always mixed species displays, which is quite entertaining for the visitor.
Not really a zoo, but the Horniman Museum, which has a natural history museum, children’s zoo and butterfly house, also has a lovely aquarium in its basement, with paludariums, tidal tanks and live corals. Chester’s is nice in terms of species list, but very boring and even poor in terms of exhibitry.