Germany and the Germans have a military reputation in rhe modern era, but historically it is based on very different things.
Ancient Germans were dangerous, according to Roman writers, because there were so many of them. "Germania" where they lived covered modern Germany, Austria, and most of eastern Europe north of the Balkans and east to Byelorussia - and "Germans" included ethnic groups from southern Scandinavia like the Goths as far as the Romans were concerned.
After a few centuries of Roman contact, the Germans had steel swords, metal armor, and a lot of military expertese they'd learned as Roman troops but you can't really point to anything specifically "German" that makes them militarily superior - there are still a lot of them, is basically it.
German knights in the Medieval period, once they are separate from Carolingian Miles, consistently got whipped by French knights, so not a good basis for a UU. The first distinctly German and Unique (and capable) Units are the Landsknechts, founded by the HRE Emperor Maximilian in 1486 - 7 years before the Spanish Colunelas that became the Tercios.
After that, Renaissance Reiters - pistol-armed "light" or half-armored cavalry, Prussian (and other German states like Hanover and Hesse and Bavaria) Grenadiers, Prussian Hussars that were the first light cavalry that could make a charge en masse like heavy cavalry, Jaegers that were the first 'regular' troops armed with breechloading rifles, the Krupp/Skoda massive 'siege' howitzers of WWI, Stosstruppen or Assault Troops that finally broke open the trench systems of WWI, the Stuka, Panzer, Panzer Grenadier, or Brandenburger (early Special Forces, a few years before any Commandos, Rangers, Spetsnaz and other later versions) Units.
BUT I would argue that the 'German military excellence' is better served not by specific UUs, but by a Unique District: the Maneuver Area.
Wildflecken, Grafenwoehr, Sennelager, Munsterlager - starting in the later 19th century (Industrial) the German military (including states like Bavaria) set aside large areas of open country where large military units like brigades and divisions could practice moving, deploying, battle actions, and live firing as units in peacetime. This gave them a huge advantage over other country's units hat had no such practice with units (usually) large than a battalion or regiment.
So, a Maneuver Area would replace the older Encampment, would have to be built separated from any other District, but gives a Free Promotion to any Unit built in them.
And by the way, taken by themselves as combat machines, most of the panzerkampfwagen were mediocre: what made the panzer units deadly was the individual crew and unit training they received, which is better represented by the Unique District than a single Unique Unit.
The other Unique that could be used to showcase "military excellence" would be the Grosser General Stab, or Great General Staff, the corps of highly-trained officers that made all German units move and act faster and with more precision and coordination than anybody else's from the 1860s to the 1940s.
I don't mind U-Boat or Panzer, but if they decide to add another medieval leader, I hope they also bring in a medieval cavalry to match the leader.
