Plusle and Minun

Plusle and Minun have been known to be a combo since the 3rd Gen. When in a Doubles together, their abilities boost each other Special Attack, making them very dangerous if used correctly. However, their HP and physical Defenses are rather low, making them easy to KO.

Abilities:
Plus: When another Pokemon on your field has Minus, Special Attack is boosted by 1.5. Made to go with Minun.
Minus: When another Pokemon on your field has Plus, Special Attack is boosted by 1.5. Made to go with Plusle.

Positive Spark

- Thunderbolt
- Discharge
- Grass Knot
- Nasty Plot/Agility
Item: Miracle Seed/Grass Gem
Ability: Plus
EVs and Nature: Modest (+SpAtk, -Atk); 4 HP/252 SpAtk/252 Spd

Negative Plug

- Thunderbolt
- Protect
- Nasty Plot/Agility
- Baton Pass/Thunder Wave
Item: Leftovers
Ability: Minus
EVs and Nature: Bold (+Def, -Atk); 252 Def/4 SpAtk/252 Spd

To be used in double battles, the only place where their abilities actually do something. Plusle has got higher Special Attack, so it's the main attacker. Thunderbolt for a powerful STAB, and Discharge for a less powerful STAB that hits everything else on the field except your Minun using Protect. Grass Knot covers Ground types, and Plusle's held item boosts the strength of it. The last move is for a stat boosting move. Take your pick. Nasty Plot to double Special Attack, or Agility to double Speed. Minun has the (slightly) higher defenses, so it runs mostly support, with Thunderbolt to use when Taunted and because even if Minun is used for support, its ability still needs to be taken advantage of. Protect, like I already said, protects Minun from Discharge. The third move is for a stat boosting move. Same as Plusle, except if you choose Baton Pass for the last move, it can be passed on to another Pokemon. If you prefer not to use Baton Pass, you can use Thunder Wave instead. Remember, use Plusle and Minun together, or else their abilities, Plus and Minus, won't boost their Special Attack.

Other Options

Volt Switch allows for scouting and getting out of unfavorable situations.
Thunder on Plusle and Rain Dance on Minun is a possible strategy.
Swift is notable for being the only never-miss move to hit both opponents in Doubles.
Entrainment used on a partner by Minun, and then Minun Baton Passing to Plusle can lead to some interesting combinations, especially with Pokemon with bad abilities, like Slaking or Archeops.
Sweet Kiss confuses a foe, although I would stick with paralyzing with Thunder Wave.
Wish can be used by Minun to not only Baton Pass stat boosts, but also to recover the HP of the next Pokemon.

Double and Triple Battle Options

Plus, Minus, and Discharge.

Partners

The Klink evolution line can have either ability, Plus or Minus. You can use it to combo with Plusle or Minun. It would be a good option for Minun to pass its boosts with Baton Pass on to Klinklang.

Counters

Since Plusle and Minun are only competitively viable in Doubles, their low physical Defense and Electric typing mean an Earthquake will easily OHKO both of them. A Grass type will resist Electric type moves and Grass Knot, walling the two.