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Adaptability and Simple don't make much sense considering weather is Castform's gimmick. I prefer the regular weather abilities (option 1) as opposed to the Primal versions. Weather doesn't see much use in Gen 6, so this could give those playstyles some extra tools.
The weather abilities (of the normal variety) win! Now onto the all important stats. Feel free to do individual spreads for each M-Castform or to give them all the same. Up to you (and good luck).
As a mon that's got a much better special movepool than physical I think it doesn't need much attack at all. So let's move most of its attack to other stats so it can hold its own against other Megas a bit better.
HP: 70
Attack: 10 (-60)
Defense: 85 (+15)
Special Attack: 145 (+75)
Special Defense: 95 (+25)
Speed: 115 (+45)
Might edit it more later. Sun and hail take a more offensive approach due to their poor defensive typing. Swapped their defenses for flavor, like Fire being nonphysical and fragile and Ice being hard and durable. Rain is the bulkier version because Water is a more useful defensive typing. I was debating about making them all bulky, but I feel that the sun and hail Castforms would still have issues surviving and switching in. Too few resistances, vulnerable to hazards, no recovery, etc.
I feel like this is the most basic spread it can probably get while still being balanced. Even though the weather abilities aren't as strong as they were in Gen 5, Castform's great coverage options and guaranteed Stab Weather Ball are enough to warrant caution. I think it makes sense for it to be more of a wall breaker so I haven't invested too much into defense.
I do really like the idea of alternate stat spread, but I think that will really affect their viability, as Fire and Ice already are at a disadvantage in terms of defensive typings. If they're all using the same somewhat Glass-cannon spread, its more of a choice based on stab and which coverage options you would need to fill out your weather team.