Uuggghhh!!! Magic the Gathering! Blashpemy! Card Crack CCGs are the bane of role-playing! I've watched one to many players waste their life savings buying cards in order to keep up with the Jones. I was a Magic addict years ago, to but saw the error of my ways and since renounced the game.
Seriously, my favorite cards when I played (Which was over five years ago so I'm not familiiar with all these new sets) were: Baron Sengir, Polar Kraken, Craw Wyrm, and Craw Giant
Really, any of the summon Legend cards would make interesting NPCs (Maybe using the Epic Level Handbook for some).
I'm just surprised that people still play Magic, after all these years and after the fact that it's obvious that to continue playing you have to keep buying more and more cards (As more cards come out, older ones are declared invalid. I was shocked to learn half the cards I collected when I played years back were no longer declared "tournament legal".)