Have visited Prague and Antwerp, but not Planckendael, which so far as I can tell is where the bulk of KMDA's strongest areas in this category are to be found.
Below are some species lists, per ZTL, for anyone interested:
KMDA:
Addax (P)
American Bison (P)
Asian Elephant (P + A)
Barbary Sheep (P)
Cape Buffalo (A)
Chacoan Peccary (P)
Eastern Bongo (P + A)
European Bison (P)
Giraffe, generic (A)
Grevy's Zebra (P)
Guanaco (P)
Indian Rhinoceros (P)
Kirk's Dik-dik (A)
Kordofan Giraffe (P)
Mhorr Gazelle (P)
Natal Red Duiker (A)
Northern Warthog (A)
Okapi (A)
Przewalski's Horse (P)
Reeve's Muntjac (P + A)
Rocky Mountain Wapiti (P)
Saharan Dorcas Gazelle (P)
Somali Wild Ass (P)
Southern Vicuña (P)
Sulawesi Babirusa (A)
Tajik Markhor (A)
Visayan Warty Pig (P)
Total: 26 species
Prague:
Addax
American Bison
Asian Elephant
Barbary Sheep
Beisa Oryx
Bharal
Black Sable
Blesbok
Burmese Brow-antlered Deer
Chacoan Peccary
Chinese Goral
Chinese Reeve's Muntjac
Eastern Bongo
Eastern Kiang
European Bison
European Forest Reindeer
European Moose
Grevy's Zebra
Guanaco
Himalayan Tahr
Hippopotamus
Javan Rusa
Kafue Lechwe
Kirk'a Dik-dik
Kulan
Lowland Anoa
Lowland Brazilian Tapir
Malayan Tapir
Mhorr Gazelle
Mishmi Takin
Nile Lechwe
Przewalski's Horse
Red River Hog
Rothschild's Giraffe
West Caucasian Tur
Western Sitatunga
Total: 36 species
Prague has a larger species list, and with more rarities - the Kulan, European Forest Rendeer, Chacoan Peccary, Javan Rusa and Beisa Oryx. The KMDA also have some nice species, like Natal Red Duiker, Babirusa or Mhorr (but Prague also hold the latter), however not quite as impressive.
As I say, I am not too well-versed on what Planckendael has in this category, so I may have to revisit my vote this evening once I have done a more thorough search of the gallery, but I will say this. Prague's exhibitry in this category is consistently world-class, with the gorgeous trio of mountain bovid exhibits, each near vertical drops of rocky ledges reaching over 200 metres above sea level, as well as the sprawling, green African Savannah and the Sitatunga enclosure, one of a rare few habitats for this species that represent their wetland home accurately. The manner in which the ungulate collection is displayed is also incredibly charming, with the gorgeous woodland setting of the zoo's upper half making for some splendid scenery and immersion as you walk through it. Their reintroductions of Przewalski's Horses also deserves much praise, as does how with the new 'Gobi' exhibit, they have managed to theme displays around their conservation in a graceful manner. The zoo's only real blemish here is the lack of grass in the hippo enclosure.
Antwerp also has a lot of very good here: excellent work with Okapis over the years, the Buffalo Savannah (a very effective repurposing of a historic building), and the gorgeous architecture of the historic Egyptian Temple. But then there is a lot of really rather poor as well. The rhino and elephant enclosures are awful, particularly the latter where if two bulls stand in the same 19th Century stall, they cannot even move without crashing into one another.
For now, voting 4-1 to Prague - may change to a 3-2 once I have further looked at Plackendael's offerings beyond the very surface level overview that I have had so far.
Below are some of my own photos to exemplify Prague's excellence here.
(Barbary Sheep cliff from below)
(Barbary Sheep cliff from above)
(Sitatunga enclosure)
(Part of the European Forest Reindeer enclosure)
(Przewalski's Horse new enclosure)
(Addax enclosure)
(African Savannah)