Limonadier is hands down one of my favourite bars in Berlin. Great service. Friendly staff. Very good drinks. They, among other things, make their own coffee liquor, which comes to great use if you order a classic white russian, Lebowski-style on request. If I lived around the corner I would also attend AA-meetings.
Oh, and another thing. It is the only bar I ever stumbled upon in Berlin that allows smoking, but also have good enough ventilation to keep even the most obnoxious non-smokers happy.