Zekrom, didn't want to have to keep trying really hard to catch the right nature and restarting the game too many times. So cheated with and Abra with an adament natures and the ability synchronize. It would have taken too long to capture hime through normal ultra balls and such...
My second Master Ball was used on Kyurem, I didn't know he was going to be lv. 75 ^^;
My current team was only lv. 60-67 at the time and I really wanted him.
(Also before anyone says anything, You get a second master ball in pokemon white/black for doing 50 trades over Wi-Fi from an in-game guy who checks how many trades you've done. After 50 trades he will give you one as a gift.)