(NOTE: This will only work in Pokemon White 2. If you own Pokemon Black 2 you'll have to trade for Reshiram.)
The white twin of jet black Zekrom, Reshiram is a potent dragon-type pokemon that can make mincemeat of most run-of-the-mill beasts. Though the star of Pokemon Black originally, it makes a return appearance in Pokemon White 2 - and you can catch the thing, with some work.
Go to Victory Road after defeating Ghetsis (which is more or less mandatory) and climb to the top. On the final hill, if you stick to the left side of the screen, you'll see a Zoroark standing in front of a cave. Follow it in and tag the creature until it leads you down to Team Plasma's old castle. Inside you'll find N, who will bring out Reshiram for you to challenge. Beat the beast - but don't try to catch it just yet. Defeat Reshiram and N will reduce it to a stone, one of the key items from Pokemon Black and White, and give you the stone.
Once you have access to Icirrus City, climb to the top of Dragonspiral Tower. At the top the stone will revert back to Reshiram. Now you can fight and capture it as you would any legendary.