Well, from what I can tell, Mewtwo (on 1st and 3rd) is BY FAR the hardest to capture in a Pokeball, he just will not stay in, I have thrown 99 before at him with 1 health (just a sliver on RBY, since there is no way to know one health) and asleep, and he JUST WON'T STAY IN. Ray seems to be next up, than Ho-oh, than Lugia. Lugia was harder to capture in second generation though. I guess the answer would be it is not extreme luck, if he is at one hp and prz/slp, and you throw 20-30 Ultra balls, I don't see him breaking out of all of them. It might cost a pretty penny to bring him home, but than again he is far and away the best second generation Pokemon, so he is worth it. Save the Masterball for one of the beast's. It is extreme luck to capture a legendary without any damage or status effects with anything less than a Masterball, but it can happen. (My Moltress on my first FR playthough, I just used a great ball for fun at the very start, he stayed in) Just keep trying, you should get him soon.