Ostrruppen Commander
After the recent buff to Osts, it seems like they offer a new play style for Ostheer.
Luftwaffe Supply
I bought it a long time ago, before it got super broken for a few months.
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Lightning War
Fun fact - in the German version of the game, both this and "Blitzkrieg" have literally the exact same name: "Blitzkrieg." They also share some abilities. #CommanderBloat. Mainly to have an extra Tiger back when I didn't have access to Opel Blitz.
Mechanized Assault
Assault Grenadiers offer a new way of playing the game early game. Also, Tiger.
I've never felt the need to get any US, OKW, or soviet commanders, but then again, I got Rifle Company in a drop, so that may explain that.