Some thoughts but with the caveat that I have only visited Budapest.
I guess Budapest will score slightly better in this category and for two reasons:
- they have some recognisable mammal species in this category: Barbary sheep, Chilean Chinchilla, Chinese dhole, East Bolivian Highland Yellow-toothed cavy, European Mouflon, Mongolian Gerbil, Nepalese red panda, Shensi Takin, and to a some extent also Siberian Pallas Cat, New Guinea Ground Cuscus, Southern Pudu and Tasmanian Wombat.
- both iconic Great and Small Rock (1909-1912) allude perfectly to the “mountains” theme, with an altitude of 34 and 22m respectively. I think they are underrated, whereas they are imposing structures that dominate the central part of the zoo. There are several wikipedia pages that pay a lot of attention to them, and I think that is completely justified: the winding paths around and over these rock formations are very enjoyable, and the large trees and the greenery in general on the slopes contribute very much to the atmosphere. Together with the few emblematic buildings, these rocks are for me the essence of Budapest Zoo, much more than the (large) collections. They belong to the same category as the ones in Vincennes and Hamburg. Should the zoo ever have the resources to give the areas around both rocks a thorough update, there are unbridled opportunities to turn them into something great. That is not the case now, as in many places in the zoo, the enclosures are rather dusty and somewhat neglected.
Zagreb on the other hand has less known mammals in this category but both voles are certainly a great bonus.
These categories are not a stronghold for birds : both have Kea and Griffon vulture as true mountain specialist. Budapest : Mountain quail (quite rare in European collections), and to a certain extent: Maronon pigeon, Hooded crow, Common hill myna.
Zagreb : Variable hawk and Ural owl.
Zagreb's reptile and amphian collections look massive, and although I have too little knowledge, it seems to have a lot of mountain specialist: Lake Patzcuaro salamander, Alpine crested newt, Mangshan pitviper, Ethiopean mountain adder, Central Turkish mountain adder,
I vote 3:2 in favor of Budapest, because both rocks are the deciding factor.