P(n) = 1 - (1 - P(1))^n
Electric / Water
Special Attack: 105
Special Defense: 107
Rotom-W is, without a doubt, THE most popular Pokemon in VGC2014 (even more than Charizard if thats possible), and for good reason. It has a brilliant typing of Water/Electric, the ability Levitate to nullify its Ground-type weakness, and access to the powerful offensive moves Hydro Pump and Thunderbolt. It has reasonable bulk; with the right investment it can survive almost any attack. It can dish out the pain versus common VGC Pokemon; few Pokemon outright resist its stabs. It can cripple attackers with its support moves. Frankly, it is one of the most threatening Pokemon in the metagame because it is a reliable jack-of-all-trades.
The few areas that it does have issue with: Its middling speed isn't that great, so it is relatively easy to outspeed. Its offenses, while good, are also able to be played around. Its defenses, again while good, only allow it to check most Pokemon and not outright counter; most Grass-type Pokemon are outright counters for it. In the sun (likely provided by Charizard Y) its strongest stab Hydro Pump is reduced, and common ground types are immune to Thunderbolt. It lacks any kind of priority, so Prankster Pokemon like Meowstic can reliably set up Safeguard and prevent Rotom from spreading status.
Set Name: Bulky Support
Rotom @ Sitrus Berry
Modest 252 HP / 116 SpA / 140 SpDef
-Thunderbolt / Volt Switch
The EVs in this set originally allowed it to survive a Timid Latios Dragon Gem-boosted Draco Meteor from VGC2013; nowadays it also allows it to survive a Timid Charizard-Y Solar Beam. The remaining EVs let it deal as much damage from its STABs as possible. Hydro Pump is required in Wash forme, but it and Thunderbolt are its STABs of choice anyway. Volt Switch is an option to provide momentum while Rotom switches to an appropriate counter. Light Screen bolsters Rotom's already impressive special defense as well as its allies, allowing it to further sponge special attacks. Lastly Will-O-Wisp lets Rotom spread Burn status, neutering physical attackers like Garchomp, Tyranitar, Kangaskhan and Mawile, who can give Rotom trouble.
The fourth move slot and the item is flexible. Rotom can opt to hold a Chesto Berry and use Rest to form the infamous ChestoRest combo, giving it further longevity; a partner like Klefki or Meowstic can run Light Screen duty if desired. Chesto Berry also prevents Smeargle from using Spore or Dark Void against it, while "allowing" another status like burn or poison to pass; Rotom can use Rest to heal and cure the status later. Protect is always an option to prevent damage to Rotom and give its partner a chance to set up or deal damage. Lastly, it can always run a Hidden Power like Ice (to check Dragon-types and grass types) or Fire (to again check Grass types, notably Abomasnow).