Surprised no one has brought up Kingdra for discussion yet. The ubiquity of rain teams is what makes it viable in OU, and it occupies an offensive position nothing else holds due to its unique typing in combination with Swift Swim.
Kingdra @ Choice Specs
Trait: Swift Swim
EVs: 24 HP / 252 SAtk / 232 Spd
Modest Nature
- Hydro Pump
- Surf
- Draco Meteor
- Dragon Pulse
I've had the most success with this Choice Specs set. It annihilates most rain teams once their Ferrothorn is dead, and even without rain it is still a deadly attacker. Having both Hydro Pump/Surf and Draco Meteor/Dragon Pulse may seem really redundant, but in all honesty it doesn't have anything else worth using, and doesn't really need any other coverage either. When using this Kingdra, simply choose between the more powerful option or the more reliable option, depending on the situation. The EVs allow Kingdra to outspeed Choice Scarf Lati@s in rain.
I've also tried LO Rain Dance and SubDD Kingdra. They're both good, though my personal favourite is still the Specs set as the LO recoil is really a nuisance, and DD Kingdra often finds itself having to lock itself into Outrage at inconvenient times due to lacking a safer Dragon attack in Dragon Claw, and does not hit anywhere near as hard as DDNite / DDMence (though I must say, not being Genesect-weak is a really great thing in this meta).