Thousands of species did I want to see and finally saw, but some four particular stand-outs are probably:
Porpoises and dolphins at Dolphinarium Harderwijk
Koalas and echidnas at Planckendael
The giant panda at Berlin
Manatees at Randers Tropical Zoo
There are still embarassingly many animals I haven't seen yet. I guess the ones I want to see the most (in a zoo setting) are the following: Platypus, wombat, sea otter, sifaka, orca, river dolphin, pangolin, dugong, bird of paradise, tinamou and whale shark.
And maybe also cougars and Visayan warty pigs. More because of the fact that I seem to be the only zoo-goer around here who haven't seen those.