i think this thread is interesting & worth reviving, so i'll add my 5 cents...
as my home zoo, zurich, has fair contenders for personal favourite in europe/world for each category, I have chosen to pick my favourites for "zurich", "europe without zurich", and "world without europe":
indoor mixed
(zurich): masoala (imho still unrivaled as *the* best indoor immersion exhibit in the world)
(europe): vogelhaus, schönbrunn (near perfect adaptation of a historic zoo building into a modern zoo exhibit) (etosha house in basel i also find an extremely well done small sized indoor mixed exhibit)
(world): kelp forest, monterey bay aquarium (...or the outer bay exhibit, I cannot decide? both are imho unmatched in their beauty by any other zoo exhibit I know) (... I would give planet penguin, loro parque, the crown for most distinctive/inventive indoor exhibit)
indoor single
(zurich): elephant house
(europe): hippo house, berlin
(world): jellyfish, monterey bay aquarium (proof that not only large mammals can draw crowds!)
outdoor mixed
(zurich): spectacled bear / coati
(europe): ???
(world): fragile forest, singapore
outdoor single
(zurich): snow leopard
(europe): "gamgoas" lions, basel
(world): orang utan, singapore