Start with Politoed + 1-2 other water types that can take advantage of rain. Include other pokemon that gain advantage from rain while covering the weaknesses of the rest of your team. My setup when I ran it in gen 5 was typically Politoed - Kingdra - Scizor - Volcarona - Quagsire - Raikou

Time travel to gen V

>>26465436
Weather is still decently viable in VI even if it isn't the powerhouse it used to be, the nerfs to weather abilities hurt bad though.

>>26465309
How about Magnezone?

>>26465474
I never used him because I personally don't like him, he's good though.

>>26465143
Politoed
Kingdra
Mega Heracross
Kabutops
Latias
Ferrothorn

Basically you want a combination of:
Politoed
an offensive Rain abuser (Swift Swim)
something non-Water that benefits from Rain
something that walls opposing Water types
something you can use to break things that wall usual Rain team mons

>>26465143
Lead with a pok that knows rain dance, have a wall that knows it, and use moves like thunder and hurricane