No form is "the best". Each have there own advantages and then there set backs.
They are all in different tiers and serve usually a similar role. Rotom-W I guess you could consider as the best because its in the highest tier and comes with some solid coverage and a typing that makes it unique to the metagame. Rotom Frost and heat do have nice fire/ice coverage but are set back by stealth rocks. Rotom-C is great at what it does but in general grass isn't the best offensive typing.
Personally I would say Rotom-W is the best. Primarily because of its solid typing and coverage. It looks pretty beast two. Its typing gives it many resistances making it a great member to have on any team unlike the other forms that need more support.