ok so catch a feebas, which i dontkno how, boost it buety to the max and teach it some good water moves
vertanturf and slateport
in contests (diamond) or tents (ruby)
You can find the normal rank contests in Pokemon ruby and Sapphire is in Verdanturf town
Verdanturf town.
Pokémon Contests are a type of competition often contrasted with Pokémon battles and held in Contest Halls. Pokémon are judged on their condition and moves in two rounds, to determine which is the best of its category. -from Bulbapedia, the community-driven Pokemon encyclopedia
Let's see... Verdanturf Town has normal Pokemon contests. Fallarbor Town has super Pokemon contests. Slateport City has the hyper Pokemon contests Lilycove City has the master Pokemon contests. Also, if you can wait, I'd recommend only making pokeblocks with the people at the master contest hall. You get better quality pokeblocks that way. Good luck!
altaria
there's still plenty to do! you can catch latios/the regis/(raquaza if you haven't already), challenge the battle frontier, and if you haven't already-contests as a side note, contests are not suited for fighting Pokemon
Aggron and Rhydon are definitely the best Pokemon on Pokemon Ruby! Their big, strong and both are great and they're awesome!
You have to understand that there is no end to Pokemon Ruby or any other Pokemon game unless you choose to delete it. For example, there's the Game Corner, Pokemon Contests, the Battle Tower, and raising your Pokemon. Those things will never go away. So no, just catching Rayquaza does not mean Pokemon Ruby is over.
Pokemon Contests were not "invented" yet.
in Pokemon D&P i would use Garchomp in Pokemon emerald,ruby,+ Sapphire i would use sceptile