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.
Rotom wash is generally considered best because it is in the OU tier. This one also has the best defensive typing with only one weakness being grass(ground is gone because of levitate) and great bulk to go with it and is one of the best volt turners in OU. It is also amazing on rain teams being able to have hydro pump boosted or if you choose a 100% accurate thunder. It can greatly abuse the slow volt switch and u-turn strategy with scizor.
Order from best to worst:
1. Wash (water)
2. heat ( fire)
3. Cut (grass)
4. Original (ghost)
6. frost (frost)
-this list is based on usage according to smogon(which means its not 100% true as people have their own opinions).
It all depends. Rotom wash has the best typing and its in a higher tier. Although, it isn't as good as rotom heat in the sun and none of them are as good as rotom frost in hail. They all run some pretty good trick sets. It all depends on how well they synergize with your team.
They all have the same stats. It really does depend on what your team is like huh?