The famous döner place at Revaler Str. 7 in friedrichshain is legit - €5 for what might be the best döner in berlin. They roast their own vegetables daily, make fresh bread, and the sauce combinations are perfect. The spicy sauce has actual heat so warn them about your tolerance level or you'll be crying

But honestly the queue is brutal. 30-45 minutes normal on weekends. Go weekday afternoons around 2-3pm for shorter waits. U-bahn warschauer straße then short walk. Cash only obviously