Landlords in Berlin
This overview shows 101 landlords in Berlin who rent out flats directly, and 87 student residences.

If you are not in a hurry, look at housing cooperatives. There, the way to a flat leads through membership and waiting lists. This can take quite long, but it also has advantages. If you need a flat soon, your chances are better with state-owned housing companies, private housing companies and property managers, because their listings change quickly. If you can get a WBS (Wohnberechtigungsschein), a certificate that lets you rent subsidised flats, the state-owned housing companies are especially interesting, because some of their flats are only for people with a WBS.
Housing companies owned by the State of Berlin. Some of their flats are reserved for households with a WBS (Wohnberechtigungsschein), a certificate that lets you rent subsidised flats.
Private housing companies that rent out their own flats. They list them on their own websites or on the portals.
They rent out and manage buildings for owners. The list only shows property managers that publish flats on their own website.
At a housing cooperative you become a member and buy shares. In return you get a permanent rental contract: nobody can end it to move in themselves (Eigenbedarf).
Houses where only or mainly students live, from the Studierendenwerk to private apartment buildings.