In German gas stations, the worst you come across is usually Oskar Maxum or Neptun. In liquor stores, it's better to avoid anything by Traugott Simon (they advertise it as "the beer with the funny name" because "tastes like watered-down piss, but you can't beat the price" is too on the nose). If you want a cheap beer that actually tastes good, go with Oettinger. They also make an alcohol-free malt beer that is excellent.