Magikarp

Stats
Swift Swim: Doubles speed in the rain. HP:
20
Atk:
10
Def:
55
SpA:
15
SpD:
20
Spe:
80

CB Karp

Item: Choice Band Despite the fact that Magikarp's attack can't surpass 200 even with Choice Band, it can outspeed everything else in the game that isn't scarfed. This, coupled with the unresisted Bounce, can make it a force to be reckoned with late-game. If you intend to use Flail, remember that it is less powerful than a neutral Bounce if more than 20% of your HP is remaining. Even when considering Bounce's low accuracy, Flail is still worse when you have more than 34% remaining. If you don't like giving your opponent setup turns, then Tackle still beats Flail if you have more than 67% health.
Nature: Adamant
EVs: 76 HP, 252 Atk, 180 Spe
Moves: - Tackle
- Flail
- Bounce

Wall of Karp

Item: Leftovers This set attempts to make use of Magikarp's enormous defense. Although his HP is horrendous, he can still take some physical hits with enough investment. Magikarp is unique in that it is the only Pokemon who can counter a +1 Lonely Pichu's Flail. Because his speed is so helpful, Magikarp has been given enough speed here to outpace neutral Pichu. If Jolly Pichu are more a problem in your area for some reason, you can still counter them using 132 HP, 196 Def, 180 Spe, but this is generally less useful. Magikarp needs all the defensive bulk it can get, plus Pichu outspeeds everything else with just a neutral nature.
Nature: Impish
EVs: 174 HP, 238 Def, 96 Spe
Moves: - Tackle
- Flail
- Bounce

F(l)ail

Item: Focus Sash Switch into an attack that would otherwise kill you and sweep after Focus Sash activates. Although your attack is only 130, you can now fire a 200 power move off for massive damage. This set utterly fails if you aren't OHKO'd, so no extra EVs are placed there and IVs are set at minimum. Watch out for status, increased priority moves, and massive walls like the cocoons who you can't necessarily 2HKO.
Nature: Lonely / Naughty
EVs: 252 Atk, 252 Spe
IVs: 0 HP, 0 Def, 0 SpD
Moves: - Tackle
- Flail
- Bounce

Other Options:

Magikarp has no other options. This diverse Pokemon learns a grand total of four moves. Splash should never even be included, in case somehow you end up needing to Struggle.

EVs:

180 Speed lets Magikarp outspeed everything else in the tier that isn't boosted by a scarf. Since speed is the only thing he's really good at, this is a good investment. Bear in mind that in the rain, Magikarp outspeeds everything, even max speed scarfed Pichu, with no investment at all. For defensive purposes, put EVs first and foremost into his terrible HP to try and let him survive something.

Counters:

Magikarp owns you and has sex with your mother far to frequently to have a counter. MAGIKARP IS YOUR FATHER.