Mine aren't particularly challenging, but by choice I enjoy this hobby on a pretty limited budget, so it's more about the thrill of the hunt for the right cards at the right price, and spacing them out enough that the spending doesn't trigger alarms to my wife
That said:
T206 Walter Johnson (either)
50 Bowman Jackie Robinson
52 Topps Bob Feller
89 Fleer FF Billy Ripken
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