Net Balls are highly recommended, since they have a high catch rate on Bug or Water Types. I caught mine with a Heal Ball, though I was extremely lucky since Balls with a 1.0 cath rate have a 0.5 catch rate if used against legendaries...
Chat with our AI personalities
I would have a friend send you a suicune, you then send it back, now the best ball would be a repeat ball. well... the BEST ball would be a master ball of course, but... meh.
by throwing it
Have a Fast Ball with you. This ball should catch at first time. If not, stack up on Ultras.
you should do what ever you think is right try a quick ball
Train a Golbat/Crobat. Or catch Entei with the Master Ball and randomly run into Raiku for the first time (Full HP) and catch with an Ultra Ball.