I'm amazed nobody has mentioned Stored Power yet.
It's a pretty high risk, high reward team setup. If the pokemon using Stored Power doesn't receive a baton pass, SP is next to useless. This is a pretty common risk considering Masquerain's typing and stats. However, receiving a single Quiver Dance brings Stored Power up to the same base power as Psyshock. 2 Quiver Dances makes it base 140, and 3 makes it 200. This is usually with STAB factored in. 200 Base Power SP with STAB at +3? Yespls.
The following NU pokes can learn Stored Power:
Clefable
Natu
Gardevoir
Musharna
Flareon
Glaceon
Togetic
Chimecho
Swoobat
Musharna can still be easily outsped even at +3, Natu's stats are too low to make use of Quiver Dances, Chimecho is outclassed in every way by Gardevoir, and Swoobat already buffs its own Stored Power well enough with Simple Calm Mind, but the rest are viable targets for this gimmick. For funsies, I've been testing this out with Chimecho. However, this set can be pulled off exactly the same by Gardevoir, only better. +2 Gardy is harder to outspeed (only something odd like Scarfed Swellow will outpace it) and has better stats, almost point-for-point.
Gardevoir @Ganlon Berry
Trace
Stored power
Shadow Ball
HP Fighting/HP Ground/Thunderbolt/Focus Blast
Wish/Psychic/Heal Bell
Modest
252 Def
4 SpA
252 Spe
So, obviously, if Gardy can receive 2 or more QDs, she's mostly set to sweep. Sucker Punch is the main reason for all of the Def investment and the Ganlon Berry (although this was also for funsies, probably better replaced with Leftovers, Expert Belt, or Lum Berry). Sucker Punch from a fully invested Skuntank only has a 6.75% to OHKO. Wish provides critical sustainability. Priority threatens to whittle you down but keep in mind how specially bulky Gardevoir is with even just 1 QD. It also gives Gardy a support option in case it doesn't receive its QDs, as well as a non-attacking option to throw off Sucker Punch. Psychic retains some damage-dealing capabilities in the same situation in lieu of support. Heal Bell can be used to prevent status from ruining your sweep.
The choice between HP Fighting, HP Ground, and Thunderbolt is up to you. Fighting handles most Dark and Steel Types without an issue, but the main weakness to this is Skuntank. Skuntank has a chance to survive even a +3 HP Fighting, resists Shadow Ball, and is completely immune to Stored Power. HP Ground OHKOs him, but leaves you with no option to hit other Dark-types, and notably nothing to hit Murkrow or Cacturne neutrally. Thunderbolt has a 50% chance to OHKO 252 HP Murkrow even without a boost, but once again leaves you with no neutral option against Cacturne. Focus Blast can take care of all of these threats, providing you survive a Sucker Punch, but it's Focus Blast.
Here are 2 videos from me with the same set on a Chimecho:
http://www.youtube.com/watch?v=aJ-kXsCUCr0
http://www.pokemonshowdown.com/replay/nu7633497
Other options:
Flareon -
Can utilize a forgotten, but usable, 95 SpA stat, good bulk, decent special movepool, and access to Wish. No STAB on Stored Power, low physical defense, and weakness to SR, but Flamethrower is great and a lack of Weakness to Sucker Punch is even better. Item choice is the same as Gardevoir's, just make sure not to use Guts if you're carrying a Lum Berry.
Flareon @Leftovers/Expert Belt/Lum Berry
Flash Fire or Guts
Flamethrower
Stored Power
Wish
Shadow Ball/HP-Fighting/Hyper Voice/HP-Rock
Modest
4 HP
252 SpA
252 Spe
Glaceon -
Mostly the same ups and downs as Flareon, but with a good defense to complement its buffed SpD and a better base SpA, allowing it to take advantage of good all-around bulk once receiving the pass. Its movepool is a bit more lacking, basically forcing it to rely on HP-Fighting to cover Steel-types.
Glaceon @Leftovers
Ice Body
Ice Beam
Stored Power
Shadow Ball
HP-Fighting
Modest
252 HP
4 SpA
252 Spe
Togetic -
Its speed is too low to be a full-out sweeper, but it's annoyingly bulky on its own even before receiving +2 or +3 SpD. This is the Bulky Attacking receiver, working with Eviolite, a usable base 80 SpA, and a great movepool to stay in longer despite possibly taking a few hits.
Togetic @Eviolite
Serene Grace
Stored Power
Air Slash
Roost
Flamethrower/Thunder Wave
Modest or Calm
252 HP
4 Def
252 SpA
Clefable -
Yes, Clefable. I'm seriously tempted to try this. 1) No weakness to SR or Sucker Punch. 2) Access to Moonlight, far better than Wish 3) Classic Normal-type amazing special movepool, including BoltBeam 4) Usable all-around stats - more solid bulk than the others and surprisingly okay 85 SpA. 60 Spe can work with investment and 2-3 QDs. Downsides are, of course, no STAB Stored Power, Normal typing, and a distinct lack of usefulness if it doesn't receive its Baton Passes. Thanks to sickweare for pointing out Clefable is RU, I always get it mixed up with Wigglytuff. I'll still be trying this by bringing Masquerain to RU.
Something viable to think about, or just a gimmick? You can't deny the power of a 140 base STAB Stored Power at +2 from Gardevoir, but there are a million and 1 ways to counter it.