Battle Moveset

A Good Moveset for Castform would have to be this:

Attacks:

Hail
Sunny Day
Rain Dance
Weather Ball

Items Attached:

Salac Berry

Preferred Nature:

Timid

Strategy Using Castform

Castform is one of the most unique pokemon to appear in the game. It has the ability Forecast, which changes Castform's type to symbolise that weather condition.
It may have below average stats, but this pokemon makes up for it with the variations he is allowed in its movesets. This one is basically setting up a weather condition and using Weather Ball to attack your oppponent.
Weatherball is also Castform's signature move and is quite a powerful move once powered up with the weather effects.
With the weather effects, Castform will be able to cover many pokemon types, including Grass, ground, rock, ice, dragon, steel, etc.
Its not such a bad pokemon when you use the variety of moves it can learn too. It would make a good sunnybeamer or thunderdancer, depending on what you would want for its moveset.
Remember people, pokemon isn't about movesets or stats, its about having fun, and in many ways, Castform symbolises this. I personally use castform on my Ruby team and it has never let me down yet.

EV Corner:

Max out the EVs in Special Attack & Speed for Castform with any remaining in Defense

Strategy Against Castform

Castform's Defense's are both above average, so attack with it's weakneses such as Fighting, or depending on the weather; Fire, Water, Electric

Contest Moveset

A good Contest Moveset for Castform would have to be this for the Cool Contest best with Lonely, Adament, Naughty or Brave Nature:

Attacks:

Rain Dance
Thunder
Thunderbolt
Thunder Wave

Items Attached:

Red Scarf

Strategy Using Castform

If you do the attacks in this following order you should have very few problems; 1st - Rain dance, 2nd - Thunder, 3rd - Rain dance, 4th - Thunderbolt/Thunderwave, 5th - Thunder

Locations in Games

Ruby/Sapphire Given to you in the Weather Institute after getting rid of Magma/Aqua Colosseum Trade from R/S Fire Red/Leaf Green Trade from R/S

Animé Appearences

Castform has had one Animé Appearence. In it, it was at the Weather Institute and held the disc that contained all information known about Groudon & Kyogre and both Team Aqua and Team Magma wanted to get their hands on it. Episode 359: Castform & The Weather Research Institute!