Rotom-Heat is best
This could change in the future depending on how the meta shapes up, but in Gen 8 OU Singles, Rotom-Heat is undeniably better than all other forms, including the previous favourite Rotom-Wash.
The reasons for this are simple: Rotom-Heat resists Grass, Electric, Fire, Fairy, and Steel moves, all of which are (or were) extremely threatening since the release of the Isle of Armour DLC.
Here's some common threats haunting OU:
- Grassy Terrain Grassy Glide Rillaboom
- Pyro Ball Cinderace (pre-ban, also checked by Rotom Wash)
- Flash Cannon / Fleur Cannon Magearna (pre-ban, mentioning to point out how much of a threat it was)
- Tangrowth/ Amoongus / Ferrothorn (defensive, but can threaten Rotom Wash with Giga Drain or Power Whip), more common now due to Rapid Strike Urshifu
- Moonblast Clefable
- Plasma Fist Zeraora
- Corviknight / Skarmory
As you can see, Rotom Heat is the best of all the forms to provide a hard or soft check to these super common sets due to its resistances, and ability to Pain Split/Defog, while also main offensive pressure with Overheat and Volt Switch.
This is something other forms simply cannot do as effectively, and in particular Rotom Wash is highly vulnerable to Grassy Glide, which has severely impacted its usage. However, Rotom-Wash is significantly better at dealing with Hippowdown, but it's simply not enough to be used over Rotom-Heat.