The hardest 5 Pokemon to catch in the game are: Cobalion, Terrakion, Virizion, Tornadus, Thundurus, and Landorus. All of them have a catch rate of 3, and are probably the ones you should use your Master Ball on. Do not use it on Reshiram/Zekrom. It's a waste of a Master Ball, because they both have a catch rate of 45. It's much easier to go after them with an Ultra Ball.
Technically, you can catch all 5 of the ones I listed with an Ultra Ball, (like I did,) but sometimes it's just easier to use a Master Ball on them and call it a day. It's up to you, really.
Yes, you can get a master ball in Pokemon black. You get the master ball towards your last gym battle from Prof. Juniper =)
Definitely Thundurus. You can get 2 Master Balls in Black and White. You should save the second one for shinies or event pokemon.
the master ball is for zekrom in white and reshiram in black
You can't!!
u can only get one master ball in Pokemon Black to my knowledge, unless u trade with someone whose Pokemon is holding a master ball, or if u hack it, which there is no hack to.
Catch him with the second master ball you get from the guy at the pokemon center at the top after you have traded 50 times
After defeating Cyrus in Veilstone, he should give a Master Ball to you.
After you defeat the dragon type gym leader, go to the route to Pokemon league you will be stopped by the professor and she will give you the Master Ball.
giratina
you can trade a Pokemon from black or white holding a master ball.
professor juniper gives it to you
Master Ball