Pretty much certain cards that either rotated out because of balancing issues and cards that were banned in Standard that we also have access to. In addition to other such cards that are so powerful they either prompt a concession on the spot or win on their own.
***In no particular order, this is my top 10***
Perilous Myr - During Origins season, this card would literally be seen in EVERY deck. It was good because most of the best creatures in the format had 2-3 toughness.
Mwonvuli Acid-Moss - This thing had been around for quite some time before BFZ hit but because of the incentive of Ulamog being the biggest baddest thing to ramp into with few answers to it in the format, Ramp quickly took over the meta and being slower was mitigated by the tempo of the Acid-Moss. Also, since majority of the metagame was midrange at the time, anything as slow or slower than MossRamp got destroyed except for Mono R.
Ulamog, the Ceaseless Hunger - If you've got no answer to this thing you lose on the spot.
Sylvan Ranger - This card enabled the powerful strategy of 4-color planeswalkers, a deck which the format had little answers for because we lacked things like Negate and other such nonland permanent interaction.
Smuggler's Copter - This guy on the play wins games and enables madness. Too much value for 2 mana.
Reflector Mage - The more you get to abuse this card to more aggrevating it becomes to play against if your running some normal creature deck.
Liliana, the Last Hope - I remember when dropping this turn 3 on the play was GG unless your opponent could race it, which they likely won't, or remove it, which at the time before Censor was unlikely.
Gideon, Ally of Zendikar - Dropping this on turn 4 with little interaction was GG.
Fatal Push - I literally see this card in every god forsaken deck on ladder these days. Definately shifted the meta by itself! Enables control decks with B, Mardu Vehicles, BG Snek, WB Lifegain...pretty much anything in B.
"the rare battle lands" - Every multicolor deck has these.