Hm. First POTW, and it's Castform. Well, let's see what there is.

Overview:
As stated in the overview, Castform is basically a gimmick Pokemon. While its stats aren't too poor, it certainly leaves something to be desired in every category. However, its ability Forecast does make it versatile with weather effects (with Sandstorm being a possible exception). Basically, Castform is only good for weather teams, and even then it isn't too great.

Ability:
Forecast: Castform changes form and type with the weather. However, this does not include Sandstorm, so it is limited to the others. Castform will be a Fire-type in the sun, a Water-type in the rain, and an Ice-tpe in the hail. Interesting, to say the least.

Sets

Sun:
-Fire Blast
-Solarbeam
-Thunderbolt/ Hidden Power [Fighting]/ Shadow Ball
-Thunderbolt/ Hidden Power [Fighting]/ Shadow Ball
Item:Life Orb/ Wise Glasses
Ability: Forecast
Nature: Timid
EVs: 4 HP/ 252 Sp. Attack/ 252 Speed

Sweeping with sun support. Fire Blast is your STAB in the sun and hits hard. Solarbeam not only benefits from not needing to charge in the sun, it also covers Castform's weaknesses when it's a Fire-type (Water, Rock, and Ground). The other moves are somewhat filler. Thunderbolt covers Water-types that aren't weak to Solarbeam, Hidden Power [Fighting] covers Steel, Rock, and Ground-types that resist Fire Blast and Solarbeam. Shadow Ball is mostly filler, although it would hit Psychic and Ghost-types for a super-effective hit.

Rain:
-Hydro Pump
-Thunder
-Ice Beam
-Hidden Power [Ground]
Item:Life Orb/ Wise Glasses
Ability: Forecast
Nature: Timid
EVs: 4 HP/ 252 Sp. Attack/ 252 Speed

Sweeping in the rain. Hydro Pump is your STAB and hits pretty hard. Thunder gets perfect accuracy in the rain, making it amazing. Since Castform is a Water-type in the rain, Ice Beam and Hidden Power [Ground] covers its weaknesses to Grass and Electric, respectively.

Hail:
-Blizzard
-Hidden Power [Ground]
-Hydro Pump
-Fire Blast/ Shadow Ball
Item:Life Orb/ Wise Glasses
Ability: Forecast
Nature: Timid
EVs: 4 HP/ 252 Sp. Attack/ 252 Speed

Castform in hail weather. Blizzard is a powerful STAB that has perfect accuracy in hail. Sweet deal. Hidden Power [Ground] covers Castform's weaknesses to Fire, Rock, and Steel, while Hydro Pump helps cover Fire and Rock-types even more. Fire Blast and Shadow Ball are filler and give some additional coverage.

Other Options:
Castform does have some utility moves that it could utilize: Tailwind, Thunder Wave, Disable. It has some options, however, Castform does not really have the defensive stats or the Speed to use them effectively. Castform should stick to sweeping.

The held item depends on your preference. Life Orb will boost all of Castform's moves by 1.3 times, but it will not be able to last as long due to losing 10% of its max HP. If you want Castform to "survive" better, the Wise Glasses won't cost HP, but its moves will only be 1.1 times stronger. While Castform could run Choice Specs/ Choice Scarf, it would be advised not to try this with the sun and hail sets, as it has issues with entry hazards, Stealth Rock when it is a Fire or Ice type. Castform would not want to be switching around too much.

Counters:

It completely depends on what the weather is like, which will determine Castform's type and movepool. Overall, special walls are the best way to deal with Castform, as it hardly goes physical and they can wear it down with Toxic or something. Castform is vulnerable to all entry hazards, especially Stealth Rock when it's in the sun or hail. Again, Castform is pretty much a gimmick Pokemon. It shouldn't be taken lightly, but don't overestimate it either.