For me, Mesprit was the hardest to catch. I didn't want to use my Master Balls (yes, balls, as in plural. More than one) on Mesprit because I was saving them. It kept either running away for killing my Golbat (the only Poke I had w/ Mean Look and stat decreasing moves.) After about 3 hours of looking for it, I caught it...damn Mesprit.