I've been to both Dortmund and Munich, both beautiful cities and awesome to watch a football match in. I was lucky enough last year to catch the Dortmund vs Bayern game in Allianz and needless to say it was amazing. Also I saw Hannover 96 and Bayern Leverkusen also to beautiful places. But the best place to watch a match (if you don't mind spending a little money) is the Camp Nou to watch El Clásico, absolutely stunning stadium and a crazy match to watch.