Burgermeister is awesome. First of all, it is located under the U-Bahn tracks. Secondly, its building is a former city toilet location (the dark green wrought iron is your first clue... plus there's still a sign that says "Männer" on the side near where you order.) Brilliant!
The burgers are about $5 and, you know, for five bucks, they're pretty great. Lots of different toppings (my husband said his burger could use more jalapeños) but mine was great with fried onions, bacon and cheese. Fries on the side and a drinks cooler where everything is $1.50 - beer, soda, water, whatever you want.
Really fun and tasty street food.