Nest Ball = Poké Ball for legendaries. Its catch rate is based solely on the foe's level. You're all mistaking it for Kurt's Level Ball, which is based on level difference or something like that.
For catching legends, I generally use a mix, since the catch rate is so infinitessimal. Many game formulas level off for very small values, so I don't think the type of ball you use makes any difference. After >30 turns, the Timer Ball's better than the Ultra, so use it if possible. If you have an extra game and can trade all the legends in from it, go ahead and use the Repeat Ball. It's only outdone by the Timer after >40 turns.