Well it may not be the best but I run a mixed Pikachu.
Pikachu @ Light Ball
Ability: Static (Ligthingrod once it is out)
Ev Spread: 78 ATK/181 SP.ATK/252 Speed (Hasty/ +Speed-Def)
- Thunderbolt (Stab, no recoil)
- Substitute (In case of a switch or the opponet trying to set up)
- Focus Punch
- Hidden Power Ice
Hitting: Steel, Rock, Ice, Water, Grass, Flying, Bug(Ice?), Dark for at least 2x.
Has a good coverage, his speed makes sure he wont be getting outspeed much and if behind a subs it can take priorities very well. I didn't run Grass Knot cuz I think is overrated not every pokemon weak to grass is heavy and really 20 BS power isnt great when dealing with waters.
Also I opted to a mixed set because Light Ball gives a 50% boost to both ATK/SP.ATK giving Pikachu with the giving spreeds ( 247 Atk which is enought to decimated with F.Punch any steel thinking it can wall Pikachu all night all day long, and 271 Sp.Atk)
For short understimated da mouse and you are going to pay for it and hard.
I think that this pikachu with some support can become deadly, Ninjask speed boost/Swords dance and another 'mon batton passing Nasty plot to Pikachu pft... =D boost his stats to sky high.