Everyone is simply listing exhibits that they are familiar with, which is obviously all they can do. I'm the same way, as Seattle, San Diego and Denver all have multiple award-winning exhibits that I could list with all the others on this thread.
However, we shouldn't overlook countries such as Germany and the U.S., with their absolutely enormous number of large zoos. I live in Canada and there are many zoos in this country, but perhaps only the Toronto Zoo and Calgary Zoo are large enough to warrant international recognition. There are others (Montreal Biodome, Granby Zoo, African Lion Safari, etc) but none have the infrastructure or resources like Toronto and Calgary.
Australia is in a similar situation: Taronga, Melbourne, Australia, Adelaide and Perth are the big 5 urban zoos. There are some decent aquariums, parks (Alice Springs Desert Park has an awesome nocturnal house!!) and open-range zoos but overall there are only 5 names that are known by most people worldwide.
What is startling is that Canada is the # 2 largest country on the planet, and Australia is the # 6 largest country, and yet between them one could argue that there are only about 7 large, urban zoos that would be internationally known. I adore smaller collections and open-range setups as much as anyone, but they aren't as well known or anywhere close to being as popular as their urban counterparts.
When discussing zoo exhibits surely the 250+ zoos in Germany or the 220 or so in the U.S. must merit consideration. Those are only the big ones, as I'm not even counting all of the smaller wildlife parks and roadside menageries. My point is that Australia and Canada are massive nations with a puny number of zoological collections. I'm venturing on a 7 week Canada-U.S.A. road trip this summer, and aim to hit a number of new zoos that I've never been to. I've never been to the State of Ohio, but they have four quite large zoos in Cincinnati, Toledo, Cleveland and Columbus. Those four zoos have more captive animals than almost all of Canada! I am exaggerating a little, but the fact remains that there are so many enclosures to be considered on a list of exceptional exhibits that it is virtually an impossible task. Major zoos are a dime a dozen throughout the U.S. and Germany, and many of those zoos must have exhibits that are superior to anything mentioned on this thread.
I see people who live near Sydney or Melbourne proclaiming that their local zoo has the world's greatest exhibit of a particular species of animal, while there are British members of ZooBeat that are saying how Chester or Whipsnade has the world's greatest exhibit of an animal species...and I am always mentioning how Seattle has won more exhibit awards than any other U.S. zoo (apart from the Bronx). So what we all need to do is to sell our homes, drop all of our belongings off at the local pawnshop, and hit the highway for a road trip of a lifetime. Let's visit all 10,000 zoos in the world!! Hahahaha...
The best way to approach the "greatest exhibits" of the world is to choose the best within a certain country or geographical zone. I don't know if some gorilla exhibit in Atlanta, Georgia is better than one in Melbourne, Australia (even with all of its damn hotwire that I saw there last June)...or if the excellent sumatran tiger exhibit is better at the Adelaide Zoo or the San Diego Zoo. It's much too difficult to choose.