Pichu

Stats
Static: 30% chance to paralyze the foe on contact. HP:
20
Atk:
40
Def:
15
SpA:
35
SpD:
35
Spe:
60

Flail

Item: Liechi Berry This is the set that may just get Pichu banned from the tier. Sub or Endure down to 1 HP, then proceed to sweep. Flail 2HKOs every Pokemon in the tier, and OHKOs all but the very bulkiest. Thunderpunch gives you something to do while you're subbing down (and hopefully a way to damage threats enough) while Reversal gives you another option against the bulky normals, although even they are no threat unless running close to maximum defense and HP. The true terror of this set is that it is almost impossible to counter; prevention and revenge-killing are your opponenet's only options. Increased-priority users, Scarf users, the fish and the cocoons are the only things that can take out this set once it has set up. Everything else falls. Bring this Pichu out late game when you know these threats are taken care of for an unbeatable sweep. Make sure you don't put any investment in HP or you will not be able to Substitute down to 1 HP.
Nature: Adamant
EVs: 252 Atk, 6 SpD, 252 Spe
Moves: - Substitute / Endure
- Flail
- Reversal
- ThunderPunch

Special Sweeper

Item: Focus Sash / Life Orb Nasty Plot boosts Pichu's SpAtk to wonderful levels and with his speed, he is hard to counter. Thunderbolt is your primary attacking move, hitting hard with Electric STAB. Signal Beam hits those nasty Grass Types who resist your STAB, as well as hitting Poochyena and Ralts harder. HP Grass (IVs listed) is for Wooper who try to ruin your fun, while Fake Out lets you beat Fake Out / Mach Punch Tyrogue. Focus Sash guarantees you'll pull that vital Nasty Plot off, but if you're good at prediction, Life Orb can let you hit a lot harder once you do (although you won't last long if you mess up with it).
Nature: Modest
EVs: 6 HP, 252 SpA, 252 Spe
IVs: 30 Atk, 30 SpA
Moves: - Nasty Plot
- Thunderbolt
- HP Grass / Fake Out
- Signal Beam

CB Pichu

Item: Choice Band This set gives you a powerful Pichu from full health with no setup. Volt Tackle is absurdly powerful and will deal massive damage to most Pokemon. Thunderpunch backs it up nicely when you are running low on health. Return provides neutral coverage against grass types. Flail is another low-health alternative, although it isn't better than Thunderpunch or Return unless Pichu drops below 10%. HP Grass (IVs listed) is to deal with Wooper. Fake out can also be used on this set for scouting, though it is not highly recommended as you will be forced to switch the next turn.
Nature: Lonely
EVs: 6 HP, 252 Atk, 252 Spe
IVs: Atk: 30, SpA: 30
Moves: - Thunderpunch
- Volt Tackle
- Return / Flail
- HP Grass

Other Options:

Pichu has quite a few good moves that aren't at all suited to his style of play, although a smart battler may be able to use them. Counter can be a nasty surprise on the Focus Sash set. Wish and Encore are nice moves, but not easily usable by something as fragile as Pichu. Thunder Wave is kind of redundant due to Static.

EVs:

Pichu should always run max speed and (Sp)Attack, as this is what it is the best at. Its power would not be nearly so formidable without the ability to outspeed everything. A neutral nature still allows it to outspeed all non-scarfed Pokemon not named Magikarp, Feebas or other Pichu.

Counters: