Playmaker" * is adopted as a key card
Some of the more sought after (read: expensive) cards on this list are: The original five Phyrexian Praetors Ragavan, Nimble Pilferer Atraxa, Praetors' Voice Skithiryx, the Blight Dragon The Brothers' War The Brothers War was the first official printing of serialized cards, and it was hot out of the gates with all 63 of the cards on the Retro Artifacts bonus sheet included in their sketch printings
Voidmage can provide a way to soft lock an opponent pretty effectively but this requires you to have reasonable board position and good amounts of blue mana
Melcharmy cards can be countered by anything that can counter Maxx "C" Ash Blossom & Joyous Spring, Droll & Lock Bird, PSY-Framegear Gamma, Called by the Grave, and Crossout Designator with the right card to banish from the Deck, are all examples
32/110 Nidoqueen
In other words, unless you're creating game cards and want some of them to appear more than once