My vote for Spain.
Not only Luftwaffe learned things on Spain, you are forgetting the best of the moment in VVS was sent to Spain also, and the learnings from Spain lead on the later develop of VVS. Without that may be Russia weren't able to defeat Luftwaffe in the sky even outnumbering it. Remember also Type 5-10 I-16 and SB M-100 was the mayor component of VVS even at Barbarossa, same that was in Spain!! Sure modelling that aircrafts will not be a waste of time for later add-ons
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S!