Dortmund are the obvious one for the yellow wall, generally being the only club to regularly menace the detestable Bayern, and the Klopp connection.
BUT Union Berlin are the coolest top flight club, for their fans, their anti-Red Bull Leipzig protest, and the running battles they apparently used to have with Dynamo (the military/establishment team) fans in the old East Germany.
Still, the club I always root for in the Bundesliga is Werder Bremen - they were my first choice so I’ll stick with them. Mostly for their surprise league wins despite being a heavily unfancied team, and Karl-Heinz Riedle, one of the most intelligent forwards to play the game. The perennial underdog who come good.