So about Beautifly...
His base stats are:
Sp. Atk: 90
Sp. Def: 50
His ability is:
Beautifly, one of the starter bug pokemon of the Hoenn region and like a starter bug, has really low BS. Because of how low it is, it is hard to use him in competitive battling.
On top of that, his typing is common and has plenty of rivals, such as the Compoundeye Sleep Powder Butterfree, Mothim whose BS is completely higher than Beautifly and learn similar moves, and Yanmega, who is just completely on a different level.
So even I had a hard time figuring out how to use Beautifly. If you are gonna use a Pokemon, you want to find something that only it can do, after all. Why else would you use that Pokemon?
So anyways, after lots of thinking...
First, it's stats:
Atk and Sp. Atk are the highest, making him an attacker. His defences and HP are very low, so Focus Sash probably is the safest item for this guy. You'll have a hard time surviving attacks or even switching in, but if you use his x4 resistance to Fighting and immunity to Ground, it is possible.
Spd is unfortunately below the 70BS line, preventing him from outspeeding major Pokemon like Breloom and Cloyster.
His only available ability is Swarm, which isn't bad at all, because his low defence stats sends him to low HP after receiving any decent attack and especially easy to use with Focus Sash.
His hidden ability is Rivalry, but I can't really say it is too useful, because the increase in power is less than Swarm and while he has several sub-weapons, lacks the BS to use them efficiently. However, it will help seperate himself from the other Bug/Flying Pokemon.
Stun Spore - Paralyzes opponents
Whirlwind - Forces opponents to switch
Giga Drain/Energy Ball - Grass move
Bug Buzz - Important STAB move
Butterfly Dance - New move in gen 5, very powerful, but Beautifly isn't that fast in the first place and Sp. Atk is not that incredible either, so you might not get good results with just one dance.
Venom Shock - Poison move, though there is a requirement to make it useful
Psychic - Sub-weapon against Poison types
Shadow Ball - Sub-weapon against Ghosts
Attract - Last chance against certain opponents
U-Turn - Atk is the 2nd highest BS and best physical bug move
Tailwind - Speeds up by 2 ranks, faster than Butterfly Dance
Air Cutter/HP Flying - Other STAB move, but lacking power
Double-Edge - Strongest physical move
So as you can see, while he does have some sub-weapons to cover Bug, there are not that many options. On top of that, his rivals can do the same or better.
Butterfree can do Stun Spore and Whirlwind better, and Mothim can do better in the attacking department.
So you have to find a combination of moves and ability that will seperate Beautifly from the others, but at the same time, a moveset that has an objective.
So what I ended up with was this...:
Item: Focus Sash
EVs: 252 Sp. Atk + 252 Spd, or 252 Sp. Atk + Spd to outspeed certain Pokemon after Stun Spore
Although Beautifly learned a new powerful move, Butterfly Dance, the problem is that Mothim could Dance as well and even better.
So put that aside.
So instead, I had Beautifly use Stun Spore and Whirlwind instead. Mothim can not learn both of these moves and while Butterfree can, Butterfree's Atk is lower and the ability is different. This ways, you can achieve a moveset only Beautifly can do.
The way it works is to first use Stun Spore on the opponent. Then, depending on the situation, attack straight with Bug Buzz or run away and do damage with U-Turn. Even without over 100BS in Atk or Sp. Atk, Swarm can help Beautifly deal out fairly heavy damage.
If you have the chance, use Whirlwind to switch your opponent out and paralyze as many of your opponent's Pokemon as possible. Predict them switching back and use U-Turn or Whirlwind again, Stealth Rock/Spikes will help make this an advantage.
After using this a little, I'd say that it worked fairly well, as far as Beautifly is allowed considering the low BS. In its best match, it paralyzed a Gyarados and ran away with U-Turn (Gyarados couldn't move from paralysis), allowing my Suicune to go first against Gyarados (Sheer Cold). It later survived with Focus Sash and damaged a Zoroark (Focus Sash) greatly, letting my Dusknoir take it out without receiving damage.