How many ZooChatters would NOT place Arnhem (Burgers) in Europe's top 10 zoos? It seems as if both Rotterdam and Arnhem are frequently hailed as being amongst the very best zoological parks in Europe.
My mental 'favourites list' of European zoos I've visited produces about a 'Top Dozen' rather than a 'Top 10', and I can't really say which I'd exclude to produce a Top 10 - but both the zoos you mention would be in that 'Top Dozen'.
EDIT: One strong factor in the favour of both is that they both have full-scale, would-be-worth-visiting-on-their-own aquariums on site.
EDIT 2: Not too many photos from Arnhem this time, as I covered it pretty thoroughly in 2010, so took the opportunity to relax the shutter finger a little and just enjoy the zoo. Much as I always enjoy zoo photography, it is nice sometimes to just have a zoo wander.
Californian bighorn sheep
Collared peccari
Bobcat
Road runner + Burrowing owl
Ring-tailed chacomistle* + striped skunk
Swift fox
Cactus mouse
Soccoro dove + Rosefinch
Gila monster
Red diamondback rattlesnake
in the tunnel adjacent to the Desert (leading to the rainforest eventually )
Merriams kangaroo rat
Variegated rock squirrel
Hispid cotton rat
Colorado toad
Milk snake
Tarantula (red-leg)
2 insect species
Free-ranging:
Turkey vulture
White-winged dove
Lawrence goldfinch*
Red cardinal
Painted bunting
Yellow grosbeak*
Ultramarine grosbeak
Fathead minnow
* indicates that this is the only holding in a European zoo
Former inhabitants include eg Amazilia hummingbirds, ninebanded armadillo, different quail species and a wide range of reptiles (of which a few are at least still kept behind te scenes).
2 pairs of Gambel's and Scaled quails each have returned to the Burgers' Desert after an absence of 4 years. Saw them just the other weekend, lovely birds.