Hahner is opposite the Alte Oper. But I strongly recommend Cove, next to the Frankfurter Hof hotel. Eckerle on the Zeil, Sør at Kaiserstrasse 3, and Abaci at Junghofstrasse 14 are also excellent. Actually, don't miss Cove if you can help it. Edit: Ansons is also good, but not unique to Frankfurt. You've probably got one in Munich already.