Starmie w/ Life Orb - Life Orb lets you switch between Starmie's amazing coverage moves. You can run Choice Specs, but you'll constantly be switching out for only a minor boost in power.
Timid Nature (Attack down, Speed up) - You might want to go with Modest (Attack down, Sp. Atk up), but you'll lose speed challenges with a bunch of people.
Ability: Natural Cure - I don't need to explain why this is better than Illuminate, do I?
EVs: 4 HP/252 Sp. Atk/252 Speed - Max attack and Speed here, pretty standard.
Moveset:
Surf/Hydro Pump - The choice is up to you, reliability (surf) versus power (Hydro Pump). This is your main STAB attack.
Thunderbolt/Thunder - If you're going with a Rain team, use Thunder - otherwise Thunderbolt is a far better choice with 30% more accuracy.
Ice Beam - Great coverage move.
Rapid Spin - Gets rid of hazards, which is great because Stealth Rock is basically the definition of the DPP metagame.