I'll help you with both, $50,000 for my Mantle and $25,000 for my Mays.
All kidding aside, I think Mantle, (if any card is going to increase substantially), it will be him as his popularity never seems to wane.
But, besides those two, there are many cards/collectables out there that have the potential to increase exponentially for the next decade, but one must figure that out themselves to the best of their ability and roll the dice.
Good luck.
