Hey,
First of all, congratulations for your prize :p
You can save it for whichever poké you'd like, but since it has a 100% chance to cap, I'd recommend something with an extremely low catch rate (Beldum, Jirachi, amongst others), that is worth it (meaning Beldum would have to be shiny to be worth a masterball, imo :p)
You can also buy masterballs for 10 PVP coins apiece, so if you're into PVP, it's not as rare, but there are other things more useful/interesting than that in the pvp shop ^^
masterball 10 pvp coins a piece? i though was 50 pvp coins ._.