For Cobb, the one below would get my vote because it's the only example, is probably the best real photo postcard of him in-action, and was snapped in 1911 during his historic .420 batting average season.
For Jackson, unless I'm forgetting something the one shown below is his rarest postcard from during his professional days.
And for Ruth, there's this unique postcard showing Ruth, Gehrig, and their young admirers during the Yankees important 1927 season.
Wagner's 1908 American League Publishing Co. postcard deserves consideration on the Wagner side of the ledger because so few have surfaced and it shows his T206 image.
For minor league collectors, there's the T210 Kelly mascot postcard.
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