Also they needed an all-german leader that was strong enough.
Ariminius ? Sure why not but a germanic barbarian raised in Rome wouldn't be an obvious choice.
Charlemagne ? They couldn't choose him, since he was "shared" with France (and actually his empire's heartlands were France), a poor choice to represent Germany.
Othon ? Barbarossa ? Any other Holy Roman Emperor ? HRE was centered on Austria (another civ in Civ 5) and the HREmperor had much less power than an actual emperor. Maybe Barbarossa would need a buoy as well.
Frederick II ? Nice choice he was a great king, but only of Prussia, not of all Germany.
Wilhelm II ? Emperor during WWI with the results that everyone knows.
Hitler ? No comment.
Bismark unified Germany, making him the best choice.