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There were actually more Po-2/U2 built than Il-2 and Il-10 counted together.
Gotta go with the Fiesler Storch. Amazing airplane, and can literally hover with a good head wind.
Me? I have a softspot for the UC-43. Just looks like an elegant tribute to the end of the bi-wing design with a little art deco flair thrown in for good measure. And amazingly fast for a biplane design with such a small engine.