I'm quite surprised nobody mentioned Zoroark actually, since it has the potential to really mess with the opponent's head.
Zoroark @ Choice Specs
Trait: Illusion
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
- Dark Pulse / Night Daze
- Flamethrower
- Focus Blast
- Trick / U-Turn
Zoroark can be a bit difficult to use since it generally requires the opponent's team to be worn down a bit, and entry hazards can be a pain when you're using U-Turn on Zoroark, but nonetheless, it can still be devestating when used properly. STAB Dark Pulse backed by Choice Specs and base 120 Special Attack is going to hurt a lot and thanks to Illusion, it can often use this move to gain quick surprise KO's, but the merits of Illusion will be covered in a moment. Anyway, Flamethrower catches out bulky Steel-types that try to switch in on Zoroark, including Jirachi, Ferrothron and Scizor. Focus Blast can be frustrating, but it is needed to be able to KO pesky Tyranitar or Terrakion that may try and switch into you. Illusion is what makes Zoroark unique, as it lets it masquerade as another Pokémon on your team, and this can cause the opponent to very easily lose one of their key team members. Say for example you send out Zoroark disguised as Keldeo, your opponent is going to have to think very carefully, lest they predict wrong and end up with a KO'd/heavily crippled Pokémon, and baiting like this can often let the real Keldeo sweep if Zoroark successfully removes one or more of Keldeo's counters. U-turn and Trick are interchangeable depending on what you want Zoroark to do. U-turn can allow you to do some damage to Tyranitar, while maintaining some momentum, however, Tricking whilst under Illusion can completely throw the opponent of track, as they would not be expecting a Scizor to give them a pair of Choice Specs.