>Pikachu-LGPE @ Light Ball
>Ability: Lightning Rod
>EVs: 252 Atk / 4 SpD / 252 Spe
>Lonely Nature
>- Zippy Zap
>- Volt Tackle
>- HP Ice
>- Brick Break / Floaty Fall

With a Light Ball, Pikachu's attack becomes higher than Mega Mewtwo X, and only 2 points below regular Rayquaza with a Life Orb.
Zippy Zap is a physical electric base 50 move with +2 priority and always crits, making it one of the strongest priority attacks in the game, and possibly THE strongest when factoring in that almost no Normal-types get Extremespeed.
HP Ice is for Ground/Grass/Dragon types, Pikachu still doesn't have a superior alternative.
Some calcs:
252 Atk Light Ball Pikachu Zippy Zap vs. 0 HP / 4 Def Greninja-Ash on a critical hit: 492-578 (172.6 - 202.8%) -- guaranteed OHKO
252 Atk Light Ball Pikachu Zippy Zap vs. 0 HP / 4 Def Alakazam-Mega on a critical hit: 252-297 (100.3 - 118.3%) -- guaranteed OHKO
252 Atk Light Ball Pikachu Zippy Zap vs. 248 HP / 104+ Def Celesteela on a critical hit: 288-342 (72.5 - 86.1%) -- guaranteed 2HKO after Leftovers recovery
252 Atk Light Ball Pikachu Volt Tackle vs. 248 HP / 104+ Def Celesteela: 456-536 (114.8 - 135%) -- guaranteed OHKO
252 Atk Light Ball Pikachu Volt Tackle vs. 248 HP / 0 Def Jirachi: 283-334 (70.2 - 82.8%) -- guaranteed 2HKO after Leftovers recovery
0 SpA Light Ball Pikachu Hidden Power Ice vs. 252 HP / 0 SpD Landorus-Therian: 328-388 (85.8 - 101.5%) -- 12.5% chance to OHKO after Leftovers recovery
252 Atk Light Ball Pikachu Brick Break vs. 252 HP / 92 Def Ferrothorn: 174-206 (49.4 - 58.5%) -- 98.8% chance to 2HKO
With base 120 speed, its priority move destroys most things faster than it, and its other moves cover almost everything else. This thing is really powerful.