Rotom-Heat
Overview: Heat Rotom, otherwise known as the micro... fire washer, was my favorite Rotom form to use, and probably my favorite Fire-type too. That remained true even while Rotom-Wash was all the rage in OU. Its Stealth Rock weakness made Rotom-Heat much harder to use and placed far more restrictions on it than Rotom-Wash, but thanks to its unique Fire typing, it boasted nine resistances-the most of any Rotom form, almost rivaling a Steel-type. Its defensive coverage included the popular Ground/Ice combination, completely walling Mamoswine; Fire/Ground/Fairy coverage from Mega Altaria; Fire/Electric/Ice coverage from various popular special attackers, especially Mega Manectric; Fire/Flying or Flying/Ground coverage from Talonflame, Pinsir, Landorus-Therian, and Mega Charizard Y; plus Steel and Bug resistances as a bonus. So although it had one more weakness than Rotom-Wash-only Water and Rock, really-Rotom-Heat actually had the edge in terms of what it could completely wall.
Of course, it was a little weaker offensively, because Electric-types' biggest enemies were Ground-types, not Grass-types. That gave Rotom-Wash's Water STAB more offensive pressure than Rotom-Heat's Fire STAB, though I personally felt Overheat landed far more reliably than Hydro Pump. Relatively speaking, Rotom-Heat's main obstacles were Water/Ground and Fire/Ground Pokémon; most other Ground-types hardly enjoyed taking a STAB Fire move.
Of course, that was not the only reason. Besides Volt Switch, Will-O-Wisp could be considered the Rotom family's defining move. Rotom-Heat obviously ran into some overlap between Will-O-Wisp and its own typing, since it readily attracted Fire-types such as Heatran. Rotom-Wash, meanwhile, could pressure those Pokémon, making Will-O-Wisp much more effective. That was the real key.
Still, that overlap was no reason to abandon the move. Rotom-Heat's standard set was the usual Rotom-family package: dual STAB, Pain Split, and Will-O-Wisp, with Volt Switch generally serving as its Electric STAB.
From there, it could run