This is a mighty fine difficult question, and one which I can't answer. For someone to answer that it would mean they would have to travel to every zoo in the world to find out...
Anyway how are we defining best? Is it best for the animal? Best for the keeper? Best for photography? Best for educational value? Best for causing traffic jams of people*? For example I enjoyed the monkey house at Edinburgh Zoo when I visited, but I wouldn't say it was the best exhibit for any other reason; at Manchester Museum they have an empty tank to demonstrate extinction and deforestation which was certainly thought provoking, but not the best for any other reason (except education).
I doubt anyone can really say which is the best in the world, as it is limited to what they have seen and their personal opinions. If you limited it more it may make it slightly easier, such as 'best Lion exhibit in the UK', but that would still be quite difficult. So this is a question which (probably) no one can actually set a defined answer for.
*meant as a joke, and anyway I think the Tomistoma exhibit or RotRA at Chester may deserve that title 
) There are countless displays throught the complex explaining the plight of animals of the rainforest and why it's important. And who could forget the amazing film with the draw-dropping surprise at the end; where the curtains lift up and you see a pretty good replica of the African rainforest based off the film you just saw. And of course there are the animals before the main attraction (gorillas)- mandrills, guenons, okapi, red river hog, colobus, and many other smaller animals that help the visitors appreciate the existence of "creepy crawleys". 
