I've been trying to catch Thundurus, but I don't know which is the best Poké Ball, the Quick ball or the Ultra ball?
I left the wild Thundurus with 1 hp and asleep, but I don't know if it counts as one turn on the new encounters or if my previous turns carry over. I am using both types of Poké Balls at the moment, and it does not look like any are good. >.<
Despite the fact that you may have been pecking away at wandering pokemon for many, many turns, whenever you meet them, it starts a new battle, from turn 1.
This means that the Quick Ball has twice the capture rate (x4) than that of the Ultra Ball (x2).
Most legendary Pokémon have a catch rate of 3. Assuming its HP is almost depleted and it is paralyzed, and Dusk Ball is used at night or in a cave, then the chance of catching the Pokémon with each ball in Generation IV is approximated by 3 * 3.5 * 1.5 / 255 = 15.75/255 ~ 6.2%, resulting in about 16 Dusk Balls required to catch the Pokémon on average. If an Ultra Ball were used instead, then the chance drops to 3 * 2 * 1.5 / 255 = 9/255 ~ 3.5%, which amounts to about 28 Ultra Balls on average. Bulbepedia
