Reshiram & Zekrom
After beating the game, you can return to Victory Road, where the Zoroark once blocked your path, and follow the path to N's Castle. You'll run into N there, who will follow you along through the ruins of the castle until you reach his old throne room.
You'll need to fight N, who uses either Reshiram (White 2) or Zekrom (Black 2). His Pokemon is at level 70 and can be rather challenging to take down.
Once you've beaten him, he'll give you the Light Stone (White 2) or the Dark Stone (Black 2), which you then need to take to the top of Dragonspiral Tower, where you'll be able to use it to awaken Reshiram (White 2) or Zekrom (Black 2).
After you've caught them, you can return to the Giant Chasm to find the legendary Pokemon, Kyurem, waiting for you there.