Tierpark Berlin Friedrichsfelde is sadly overlooked, but well worth the short U-Bahn ride from Alexanderplatz. It is Europe’s biggest landscape zoo, and is more like a huge park with the animal enclosures spread over a large area. A lot of walking is involved, and for long stretches, it feels more like being in a forest than in a zoo. There are countless different plants, bushes and trees, a.o. many rhododendron. In recent years, it has become famous for its successful elephant breeding programme, and it is also one of the few zoos in Western Europe that houses herds of ungulates, including some rarely kept species such as Hungarian longhorn, muskoxen and takin.
The only downside, which could easily be rectified so I don’t know why that has not happened yet, is that it is incredibly badly signposted (in strategic places not at all), the map they give you at the entrance is useless, and getting lost is unavoidable. You therefore end up walking even longer than necessary, so wear your best walking shoes. The only useful map is posted just inside the entrance (but for some strange reason only seems to exist in that one copy), so it may be a good idea to take a photo of that one before starting out. Or remember to download one here prior to the visit.
Here are some photos from my latest visit: