The thing is with this question is that not every Rotom form is viable in OU. The only viable Rotom form in OU is Rotom-Wash, which is currently A- in the OU VR. Rotom-Heat is C Rank on the OU VR. The other Rotom forms are UR. Here's a quick breakdown of the roles the Rotom-forms in the tiers they are used in:
Rotom-Wash is used as a Defensive pivot in OU and UU, and can also use a Choice Scarf set in UU.
Rotom-Heat can be used as a defensive pivot or Nasty Plot user in UU.
Rotom-Mow can use either Choice Scarf, Choice Specs, Offensive Defog, or a Nasty Plot set in NU.
Rotom-Frost is ZUBL and is UR on the PU VR, but can use a Nasty Plot set.
The regular Rotom form can use a Choice Scarf, utility, or Nasty Plot set in ZU.
Rotom-Fan can be used as a Pivot or a Nasty Plot setup user in ZU.
TL;DR There isn't really a best role for the Rotom forms in the tiers they are viable in, as some can be versatile, depending on what tier they are used in.