L'Amir Lebanese specialities is incredible for shawarma sandwiches. It's located across the street from the entrance to Sachsenhausen and is a tiny stand, there is no place to sit, you get your food and stand on the sidewalk, there are two small high tables to lean on. The shawarma is offered in either lamb or chicken both are excellent as is the toasted kibbee sandwich (roll). There are 4 other similar places on the street but none compare to the quality of L'Amir. The owner is very nice and told me that his brother operates an outpost under the same name in Seattle.
One other tip for Frankfurt, near the train station is an Iranian place called Hafez, they are not orthodox Muslims as they serve beer and wine. The entrees are OK but I would reccomend stopping in for a grilled eggplant appetizer that they make, sorry can't remember the name, its slices of grilled eggplant with a mixture of yogurt, lemon zest, coriander and other spices on top. When they do it right its transcendent.