I don't buy anything for it's 'cool' factor. As such, the only things that could fall under this category for me are magic cards as I've been collecting since revised. Classic cards of note:
- Complete playset of all Dual Lands including a Mint Unlimited Underground Sea
- Playset of Gaea's Cradle, Force of Will
- Foil Promo Swords to Plowshares
My personal favorite though....
Card Text: Players must leave game in progress as it is and use the cards left in their libraries as decks with which to play a subgame of Magic. When subgame is over, players shuffle these cards, return them to libraries, and resume game in progress, with any loser of subgame halving his or her remaining life points, rounding down. Effects that prevent damage may not be used to counter this loss of life. The subgame has no ante; using less than forty cards may be necessary.
Oracle Text: Players play a MAGIC subgame, using their libraries as their decks. Each player who doesn't win the subgame loses half his or her life, rounded up.
This card is just absurd and has led to many a fun reaction when played against people who don't know about it's existence. Sadly I only own 1. Before it got banned I always wanted to build a deck with 4 of these, regrowth effects and tons of exile cards like swords to plowshares. It would be Green/White/Black (for tutors). These cards aren't shuffled back in subgames so the idea is that whittle your opponents library over multiple sub-sub-sub games. It's the kind of deck I'd want to build and play once or twice a year at best for a laugh.
In reality I ran it in my weenie white deck for quite awhile. Turn 1 Savannah Lions, Turn 2 Shahrazad.
Win the subgame and attack for 2 bringing your opponent down to 8 life! Good times!