I really like Tanne B because of their soya ice cream that is homemade and super delicious. They often do soya chocolate, vanilla, almond, straciatella... All of the fruit flavours are vegan as well. It's all marked vegan so you don't need to ask to make sure. They have finally introduced a vegan wafer too. Prices are okay for the quality you get.
In summer, they have another location in Kreuzberg, at Marheineke-Markthallen.