I'm pretty new to competitive battling, but I came up with a fun set for Starmie that I've found pretty successful in my wifi battles. Not sure how viable it'll be once the dust settles on the X/Y metagame, but it's been pretty reliable for me so far. I'd be interested to hear you all's take on it. I call it...
ComeAtMie Bro
Item: Weakness Policy
Nature: Timid
Ability: Natural Cure
Evs: 252 HP / 28 Def / 4 SpA / 224 Spe
Moves
- Scald
- Psyshock / Ice Beam / Thunderbolt / Rapid Spin / Substitute
- Recover
- Cosmic Power / Rapid Spin / Substitute
Starmie is actually a surprisingly good user of Weakness Policy. Because it has a number of common weaknesses, it's fairly easy to bait your opponent into hitting you with a super-effective attack.
With just one Cosmic Power buff, Starmie can at least one super-effective hit from most of the heavy hitters the meta-game. And with its blistering speed, Starmie is pretty much guaranteed to get at least one Cosmic Power off.
Scald adds to Starmie's survivability - if the burn procs against a physical attacker, Starmie becomes even harder to bring down.
Basically, this starts off as a stall set. You switch into something that Starmie either resists or takes neutral damage from (preferably a physical attacker), use Cosmic Power to soften the next blow, then Scald until your opponent gets burned. If and when a burn procs, you can use Cosmic Power to boost your defenses still higher, do more damage with your attack moves, and/or recover off damage.
The fun begins when your opponents switches in a counter. After Starmie eats a super effective hit from its unsuspecting opponent, its power rockets up and it suddenly becomes a lot more dangerous. Usually, this is more than enough to to OHKO or 2HKO whatever your opponent has brought in to counter you, and then you can proceed to punch a hole through the rest of his team.
The best part is, Weakness Policy activation outprioritizes Knock Off and Thief, so if your opponent comes after Starmie with either of those moves, he's in for a surprise!
Scald and Recovery are mandatory for the set. Cosmic Power is near-mandatory, but you can run Rapid Spin or Substitute if Starmie has dual screen support. In my team I have a dual screen setter, so I run Rapid Spin to give Starmie added utility even when I'm not trying to set up a sweep.
The second move is whatever coverage you need. I prefer Psyshock, as that allows Starmie to be a fairly effective wallbreaker after its Weakness Policy procs. You can also take Rapid Spin or Substitute in that slot, if you want one of those in addition to Cosmic Power. Obviously, if you do that you'll run the risk of getting completely walled by a Dry Skin / Water Absorb pokemon like Jellicent or Vaporeon.
Once this Starmie gets going it can be pretty hard to take down. It resists many of the most common priority moves (though Shadow Sneak still smarts) and can Recover off any hits it takes. And because of Natural Cure, if your opponent tries to force you out with Toxic, you can just switch out and try again later. Phazers and critical hits are probably the biggest threat. But even if you do get phazed out, this Starmie can still be useful as a rapid spinner or stall.