Lol, Giratina isn't that hard to catch, just get it down to low HP, then wall it with a Normal/Flying type (only Slash affects it). Strange how Regigigas isn't up there either. I caught mine by walling it with Spiritomb completely (only Confuse Ray works on Spiritomb, which does no damage).
I actually found Dialga the hardest (none of them where actually hard to catch), since it ws at a point in the game where nothing I had had proper moves for catching (Sleep Moves, set damage moves (Super Fang works fairly well), and False Swipe).
In Platinum, I'm guessing Giratina will be, by far, the hardest legendary to catch, since it has a catch rate of 3, and it's at a pre-E4 stage of the game.
However, in all of D/P/Pt, the hardest to catch, or rather the most annoying, would be Beldum. It has a catch rate of 3, and Take Down's recoil can KO it. Best way to do it is with Gengar/Spiritomb/any ghost with Hypnosis and Night Shade or something.