Although I am not surprised that London got a mention on this thread, I think it is thoroughly undeserved.
In 2019, it would be very appropriate to say that the zoo was in decline. The Aquarium closed, the North Bank Aviaries closed, Snowdon Aviary was emptied, long-time favourites such as Bearded Pigs and Giant Anteaters were departed, and the issue of wasted space was worsened when the former anteater and vicuna enclosures were transformed into a mini golf course. In 2020, this issue was of course worsened when the zoo was forced to close due to the pandemic and subsequent lockdown.
However, since those tremendous low points, the zoo has seen nothing but major improvement. Between SLoRA (the new Reptile and Amphibian House, indisputably the greatest in the country with regards to both its collection and its exhibitry), Monkey Valley (the repurposed Snowdon Aviary, now an excellent, although admittedly underused, colobus walkthrough) and Giants of the Galapagos (an excellent Giant Tortoise enclosure), we have received three major new exhibits. Enclosures that have been empty and underused for a long time have received new inhabitants (such as the Chinese Water Deer in the former reindeer enclosure and the Grey Mouse Lemurs in the third enclosure in the Aye-aye House which, if I am not mistaken, has been empty since its creation), and several other exciting new species have arrived, both in terms of crowd-pleasers (Ostrich) and in terms of enthusiast-pleasing rarities (the only Small Indian Mongoose in Europe). Although the majority of them were invertebrates, the past year has seen a net increase of over 50 species, which is nothing short of remarkable, and larger than some zoo's entire collections. Top that off with several exciting births among the zoo's star attractions (gorillas and lions, which will no doubt prove to be great tools for the marketing department) and rarities with conservational significance (Laos Warty Newts and Mountain Chickens), and you have a zoo that is clearly in a state of drastic improvements. To do that on a post-lockdown budget that has caused problems and even closure, while still spending more on in-situ conservation projects than any other zoo in Europe, is phenomenal.
Any claims that London is a state of decline nowadays can only realistically be in terms of comparing it to the seventies or eighties, which, to put it in football terms, could be seen as its prime. And it was in the sense that ZSL had the largest species inventory on the planet, was a gamechanger in the breeding of certain species and had every large, star attraction animal imaginable. However, if you look at any of the enclosures which said species were offered at the time, calling this its 'prime' seems a little odd - while the collection was immeasurably excellent, the display of said collection was horrific, and I hope we can all agree that animal welfare should always be prioritised over the extent or comprehensiveness of a collection. There are only two criticisms I have of the zoo at the moment - the underuse of certain areas, such as the Mappin Terraces and North Bank Aviaries (although I believe there are plans to change the latter) and the poor quality of the giraffe enclosure, which even then isn't that bad for just two females, and with the decision already made not house bulls there, we need not worry about that changing.
It was mentioned upthread that London is considered to be one of the top twenty zoos in the UK, which is beyond doubt true. However, and this may be a hot take, I would even go so far as to say that it is a shoo-in for top ten - or in my, admittedly slightly biassed, opinion, top five! Short of its potential? Perhaps. In decline? Quite the opposite!