Reshiram was part of my first team, until he hit L100. Now, he sits in the box with other L100 pokemon as I train up another team of L100 pokemon to join him (sorry, I got a penchant of training pokemon to L100. Last time my friends called me crazy because I had a L100 Raticate that I caught at L2 and was the first pokemon I caught in my FR. He's very weak though).
I got a Zekrom through the GTS for pokedex purposes initially, but then I decided to train him to 100 so my Smeargle can learn Bolt Strike (he already knows Blue Flare).