Usually they take the Pilsener Beer to mix it with lemonade for a radler. I think this is because Pilsener itself has a more or less bitter taste so the whole thing won't get too sweet. Make sure you take lemonade with very natural ingredients or you get a weird artificial taste. In northwest Germany they call the same thing Alsterwasser (like the water of the Hamburg river Alster). If you are in Bavaria you can order a Russ (a Russion). Then it is made of Hefeweizen (Weißbier) and Lemonade.