Ooh, tough one. But I'd have to say Pokerus. Harder to get than shinies since you can RNG for those once you have your SID and all plus use Masuda's Method, PokeRadar, etc., and so long as you keep a Pokemon infected with it boxed forever you can use it to infect as many Pokemon as you want.
RNG or SR luck? (Random Number Generating or Soft Reset luck, meaning using your SID to make shiny/good IVs and a good nature hunting easier or continuously pressing A+B+Start+Select/L+R+Start+Select until you have a shiny/Pokemon with good IVs and a good nature)