Being the 'biggest' club in the area I've been living for 50 years, I suggest Sportfreunde Siegen. They are playing the Oberliga Westfalen.
This club, founded 1899, won the german Amateur Championship in 1955. Played 2005/2006 1 year in the 2. Bundesliga, went almost bankrupt, raised again, went bankrupt again and now they are trying not being relegated in the 6th division.