First and foremost, there's everyone's favorite Rabbit buildaround, Hare Apparent
As long as it was in the format, it was played, and it was a core card in some turn-one decks that would just knock out your only Pokemon in play to win the game on the first turn
This used to be a complex deck to pilot, with spreadsheets dedicated to the best lines to take depending on what interaction you faced and how much mana you had
Just like the show, however, some of the synergies are just way too disjointed from each other, like the plot between seasons, to consistently win your games, unless you are a Time Lord and can manipulate the deck to its limits
Or the Auctioneer may immediately put the lot up for sale again and resell to the highest bidder
As a phenomenally popular collectable game, its no surprise that Pokemon has given us some rare, and so valuable cards over the years