It takes a very long time to provide extensive lists of species at the various zoos in this competition, but I spent 20 minutes looking through photos and came up with a few highlights in terms of exhibits.
A quick rundown of 'gems' would include the fantastic Cape Buffalo Aviary at Antwerp:
@Dianamonkey
Antwerp's White Rhino paddock:
@Valvas
There's some grassland species inside Antwerp's historic Bird House:
@KevinB
The Greek temple on top is the Reptile House at Antwerp, with some grassland species inside.
Meanwhile, Planckendael has a stunning Cheetah exhibit:
@KevinB
There's an equally spacious and lush Spotted Hyena exhibit at Planckendael:
@vogelcommando
Planckendael's Indian Rhino paddock (which also possibly still contains Visayan Warty Pigs):
@Toddy
The Kordofan Giraffe/Mhorr Gazelle exhibit at Planckendael is excellent:
@KevinB
The large 'Prairie' paddock at Planckendael, with American Bison and Elk, is splendid:
@KevinB
Zagreb has a spacious African Aviary:
@twilighter
There's also a Lion exhibit in Zagreb that looks to be decent:
@twilighter
I voted
4-1 for the Belgian zoos.