Hardest to find: Kangaskhan - because you have to check the Great Marsh every day until you see him, not to mention trying to capture it.
Hardest to capture: Beldum. No arguement. Catch rate of 3 on a "normal" pokemon is rediculous. I mean, the Dimension pokemon (Dialga and Palkia) both have catch rates of 30! I caught Dialga on Platinum version with one Quick Ball without weakening it, but it took an average of 20 dusk balls to catch one beldum that was asleep with 1HP.
If you're seeing this post, it must be old, because this place has turned into a total wasteland of non-interesting threads and rude people.