In game, it doesn't really matter.
Pretty much an y Pokemon that you've trained can be a sweeper. Even a Magikarp. All it takes is to grind a lot.
If you're looking for an easily accessible Pokemon that doesn't need a lot of training to sweep, here are a few suggestions.
With a high Sp. Attack and high-enough Speed, it will be able to sweep most Pokemon you meet throughout the game with STAB moves such as Psychic, Psyshock, and Moonblast. It also has a Mega Evolution. Its first evolution, Ralts, can be found on Route 102.
Same as Gardevoir, with a high Sp. Attack, but this time he has lower Speed. This should not be an issue though, as his defensive stats are competent enough to tank a few hits. Just watch out for Water-types as he is x4 weak to them. He also has a Mega Evolution. Its first evolution, Numel, can be found on Route 112, Fiery Path, or Jagged Pass.
Milotic can be harad to get, but is very rewarding once you have one. With amazing Sp. D and high Sp. Attack and Speed, and a good movepool such as Hydro Pump and Mirror Coat, Milotic is a very good Special Sweeper. Unfortunately, Milotic does not have a Mega. ;-; Its first evolution, Feebas can be found on Route 119 under the bridge while raining, I believe. It evolves by having Max Beauty stat and leveling up.
Starmie is pretty much the same as Gardevoir, statwise. It has a decent movepool, and is relatively easy to obtain. Its first evolution, Staryu, can be found in Lilycove City. You need to use a Water Stone on it to get a Starmie.
This isn't it, either. There are plenty more Special Sweepers that can be used, I just picked out the few that I thought were the easiest to obtain and simplest to evolve. Hope I helped!