The Burgermeister (Burger-Master) has some great fries and good burgers. It is situated in one of those old public toilets under the U-Bahn rails of the U1 so left and right of you all those cars are passing by, while you can also see the flow of people going in and out of the Schlesisches Tor U-Bahn station and behind you is the Oberbaumbrücke and the river. It doesn't get more urban Kreuzberg than this. I had a Tofu-Burger which was yummy and very fair for 3,50€.