Idunno guys, you dont give him enough credit.
Magikarp @ Choice Band
Ability: Swift Swim
252 Atk / 252 Spe / 4 HP
Adamant Nature
-Tackle
-Flail
-Bounce
-Splash
This moveset utilizes Magikarp's ability Swift Swim, which doubles his speed in the Rain. Flail and Tackle would normally be walled by ghosts, but thanks to gen 4 he got Bounce, so he can hit ghost types. Choice Band buffs these moves up even more. In rain, this moveset can be quite devistating.
Magikarp @ Leftovers
252 HP / 56 Atk / 200 Spe
Impish Nature
Ability: Swift Swim
-Bounce
-Flail
-Tackle
-Splash
A bulkier version of the above Magikarp, bounce gives you a turn of immunity and extra leftover's recovery. Flail and Tackle give you decent coverage. Splash is great for PP Stalling at the end of matches.