Ariados is another one of those bad Bug Pokémon you find at the start of the game, but it can actually work well in a UU battle. That decent attack power, coupled with two great attacks is sure to screw a few Pokémon over, it can Baton Pass a few nice things to other Pokémon to benefit from, and it completely stops Hypno, Parasect, Jumpluff, and friends from mutilating you with sleep moves (temporarily). Yes, it is a weak Pokémon, but use it right and it can make quite a positive difference to your team.
Here we have a Pokémon with the same stats as Mightyena, only slower. That's not good, but Ariados can be an effective starter and physical sweeper if you use it correctly. Double STAB is always good, particularly with Signal Beam (Bug attacks come in short supply, you see), and with that decent Attack, Ariados can do some good damage if it hits the right Pokémon. A 10% chance of confusion is always handy as well.
Start off the battle with either Spider Web or Agility (Agility is generally better), and start attacking. When Ariados is close to fainting, you can Baton Pass the Speed or trap onto another Pokémon to benefit from. It's simple, and quite effective; just watch out for its weaknesses, like its old buddy Golem.
Ariados also works well enough primarily as a Baton Passer. Try using both Agility and Spider Web, attack with Signal Beam, and pass on the goodies. Also, always remember that Ariados does not fear sleep moves thanks to Insomnia, so predict your moves right to cancel out that stuff.
Substitute is something else that can be passed (Double Team can also be passed if evasion moves are allowed). Giga Drain can be used for Golem, Sandslash, and the other Pokémon that resist its Sludge Bomb. Ariados can also Toxic and trap the opponent, although it's a strategy that generally doesn't work. Thief and Hyper Beam are your standard suicide moves, really, and both work well enough on this Pokémon.
Well, if you're going with the physical sweeper set, you will want to boost that Attack a lot. However, put a fair bit into that HP as well, so it can actually Baton Pass something as well. If you are using it primarily to Baton Pass, its defenses should be your area of focus.
Checks and Counters
Both defenses are horrible, and none of its attacks (even Giga Drain) will do serious damage to Golem, Sandslash, and other hard skinned Pokémon resistant to both STAB attacks (who usually hold Rock Slide anyway.) Also be wary of fast sweepers like Fearow, Mr. Mime, and friends. Whether you have Spider Webbed it or not, you will need to get Ariados out of there. Steel-types are immune to Sludge Bomb and resistant to Signal Beam, so they are also excellent counters, particularly those with Roar or Whirlwind in case Ariados gets a Spider Web or a few boosts.