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nbrazilmost popular pre war set: T206
most popular card type in T206: portrait
most popular pre war player: Ty Cobb
most available (and least expensive) portrait type of Ty Cobb: Red
My guess: T206 Ty Cobb Red Portrait.
I guarantee you other Cobb cards would be owned more (the e cards specifically)...but price and scarcity always get in the way.