I have both Pearl and Platinum, and each one had a varying catch rate each time I played each game.
Palkia (Pearl, 1st time): Easy. Caught Palkia w/in a few Ultra balls.
Giratina (Pearl, 1st time): Took a few tries, unintentionally captured it in a Poke ball.
Palkia (Pearl, 2nd time): By this point I understood natures a little more, so I just used my Master ball.
Giratina (Pearl, 2nd time): Traded with a friend for another Master ball and used that.
Giratina (Platinum, 1st time): Giratina-O just would not stay in any sort of ball! Used master ball
Dialga and Palkia (Platinum, 1st time): Both were easy. Caught Dialga in a Dusk ball and Palkia in an Ultra ball.
Giratina (Platinum, 2nd time): KO'ed Giratina-O and caught it in the Turnback cave. (can't remember which ball I caught it in)
Dialga and Palkia (Platinum, 2nd time): I think I had two Master Balls (one from in-game, one imported from Sapphire) and just used it on them both.