Hearthstone is the only card game I'm familiar with and seems to be underrepresented in here so I will post some cards that used to be OP and some ones that are widely considered to still be.
This combo was widely considered busted as fuck, as against Hunter people were afraid to play minions, which against hunter is a death sentence. Buzzard was changed to a 5 mana 3/2, effectively killing it forever (thank fuck) and UTH was changed to a 3 mana card, which everyone still uses.
You can have a max of 10 mana (save certain situations) in Hearthstone. Leeroy is a legendary, you can only have 1 copy of him in your deck. The Miracle Rogue deck would commonly play Leeroy, Shadowstep Leeroy, then play Leeroy again, then Shadowstep him AGAIN for 18 damage and still have two mana left for a hero power or Eviscerate. This was obviously bullshit, so Leeroy was increased to 5 mana so the combo didn't work anymore.
Probably the most despised card in the game for a long time before it was nerfed to gain just 1 attack, a good opening hand using Deathrattles pretty much won you the game, which is extremely frustrating.
What cards are still broken?
Mad Scientist costs 2 mana. Paladin secrets cost 1, which is still great value. Hunter secrets cost 2, which is insane value. Mage secrets cost 3, which is fucking stupid as hell value. This guy has been a problem for over a year now.
This card can instantly win you the game from a pure diceroll, which is stupid. There is NOTHING more frustrating then losing to a lucky Unstable Portal which gave the opponent the exact card they needed to win.
AKA Dr. Balanced, this is widely considered to be the strongest card in Hearthstone now and is so strong it has changed the meta around itself and the only reliable neutral card counter, Big Game Hunter, which some consider to also be OP because every decks runs it and any minion above 7 attack is now dead.
On first glance it didn't appear that strong, but it soon became pretty obvious that that 1-4 damage could turn the game around completely.The little 1-1 suicide itself into a 5 health minion, then killing it with the 4 damage from the deathrattle became one of the most annoying things to see in the game.