Erif
...is magical
Mathematically? What the hell are you talking about.You guys, mathmatically the Regis, Latis, Rayquaza and Deoxys are the hardest to capture. Oh, and Beldum if you encounter it in the wild. (Which isn't possible until D/P/Pt, but yeah.)
I think all the pokemon in this game are pretty easy to catch. Latis are hard to find, what they're fairly easy to catch once you do find them. But maybe Suicune from Colleseum...