In my opinion 1952 Topps Mantle. Everyone knows it and most want it. And, it's a great looking card. The T206 Wagner is unobtainable for most and it's a fugly card, Alfred E. Neuman look alike
Much more of the general public could pick the Mantle out of a lineup versus the Wagner. In the card world people know the Wagner, elsewhere not at much. When I owned the Wagner below, friends and family outside of the hobby had no idea what it was. I could show them a 1952 Mantle and everyone knows Mickey Mantle, even if they don't know exactly what year it is.
Pics of both of them for context....I have a good eye for 2's
Further consideration should be given to of all things the 1968 Topps Nolan Ryan Rookie. When the hobby was exploding circa 1990+/- it was the hottest card on the planet and still is in high demand in all grades. Like the Mantle high demand and ample supply make it a marketing darling. The Ryan Rookie was the gunpowder for the collecting boom of the 1990's.