If you were a Gym leader and had to choose a type

[KoA]

I would suggest any water team to have a Kingdra. He's pretty much THE Rain Dance Pokemon.

Having a pokemon with Damp Rock use Rain Dance will give you 8 turns of rain. Utilizing this with Kingdra's Swift Swim and enhanced Water attacks under the rain, you have an all out force of destruction who outspeeds pretty much everyone and does loads of damage. I'm pretty sure a supercharged Surf OHKOs Gengar. I know Hydro Pump sure as hell does.

Ludicolo is also beast under the rain.

[Omega]

Water type

-Empoleon
-Kingdra
-Kabutops
-Miltolic
-Wailrein