My friend has used this set on me multiple times, and when it gets the chance to set up it can be quite a beast. Normally he uses it as a late-game sweeper, but it works as a lead as well. I've been talking for a bit too long, so here it is:
Beautifly @ Focus Sash
252 SpA / 252 Spe / 4 HP (it might be def/spd, i'm not sure)
Modest/Timid (again, i'm not sure)
Trait/Ability: Swarm
Quiver Dance
Energy Ball
Bug Buzz
Psychic
I know, I know, Beautifly is technically in NU but trust me, this set can work.
The concept isn't too hard to grasp; you set up with QD, and wreak havoc. Swarm plus Focus Sash works really nicely with this set; QD until the Sash is activated, or you get to really low health, and then the boosts in Special and Speed pretty much do the rest. Plus, the Swarm boost helps too.
Check-and-counter wise, a special wall that resists or gets neutral on it's attacks (such as a Togetic with Eviolite; its defenses can take the hit, and it can return fire with a STAB air cutter even after QD, since it walls Beautifly's attacks anyways) usually force it out, and entry hazards or residual damage are always bad news for Sashed pokemon. Since it makes use of a focus sash, it's easy Sucker Punch bait after the sash is activated (or any priority move, for that matter); and regardless, being an NU pokemon, its defenses aren't stellar either, so an unboosted priority move that hits for neutral or super effective damage will probably take it out anyways. (Ice shard anybody?)
This set, obviously, needs a good team around it in order to work well. Healers/clerics are always welcome, and spinners are useful for getting rid of those entry hazards. Beautifly is weak to fire, ice, rock, and flying (tell me if i'm missing anything here), so a pokemon that can take the hit and then hit back would be. Aggron would be a good example. It resists rock, the most potent threat to beautifly (unless its already boosted, then energy ball will take care of it), and can retaliate with Head Smash or Stone edge for fire, ice, and flying types, and earthquake for the rock types. In terms of walls for support, either will do depending on what you're predicting the opponent will try to KO you with on the switch out; physical for most rock and flying attacks, and special for most fire and ice attacks.
I haven't run any damage calcs for it (seeing as it's not my set to begin with) but if someone could help me out there then that would be really awesome. Don't underestimate this beautiful butterfly!