The most overpowered Splinterlands card is...

...currently a card named Gremlin Blaster.

Maybe not in the higher leagues, but it's the real killer in the silver league. With the help of weaken ability from the cards like Swamp Thing or Goblin Shaman you can potentially kill all the 3 HP monster in an instant. You don't even need to level him up because he's this strong from the first second you obtain this card.