i depends what moves you teach which Pokemon, but i would recommend a kyogre with surf, hydro pump, water spout and dive. i repeatedly won the master rank with it.
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A little girl will give it to you in the slateport pokemon contest room
Feed best pokeblocks to your pokemon, Teach best moves for the contest your entering and just use the right moves at the right time.
SLATEPORT CITY.Kimm57:Go to Slateport City, then go southwest of the Pokemon contest
you can get yourself a pokéblock case at the lilycove contest hall.
if you go to slatport and you go into the contest it says you have to go to a certain city and you have to walk to that city and if you go into that contest area you will talk to the person at the desk and she will give you a contest pass.