Of course you know the big international firms offering real estate.
You may also know some German firms, operating internationally - but you should be aware, that very often these brand names represent just franchise firms.
The quality of the single branch might differ a lot.
You should look for agents, which are members of RDM or another well reputated organisation.
Certificates, given by technical control boards do not mean too much.
You may find also, that there are some agents specialized in very special markets like properties of Deutsche Bundespost or Bundesbahn or in supermarkets.
Generally you should not hesitate to ask small, but well reputeted firms.
You may find interesting offers in the internet, searching with www.immonet.de, or with www.immobilienscout24.de or www.immowelt.de.
You can search for different areas, different categories and of course for a certain price range.
Many of the estate agents put a part of their offers into these search engines.