This can be based on sale or book value, depending on which you feel most comfortable with. Also feel free to share how you acquired the card (trade, gift, purchase, etc).
Without a doubt my most valuable card is a 1961 Topps #300 Mickey Mantle. It was acquired via trade from RGBII (thanks for helping me get this). Beckett book value is $600, although mine is worth less due to condition.
(Yes, I realize I incorrectly labeled the file "1962".)