Let's get the obvious few out of the way. Air Slash is unquestionably terrible, STAB Leaf Storm doesn't cover much except for Swampert, who has no reliable recovery and thus can't take many Shadow Balls. Blizzard is not that bad, but it is unreliable, so Hidden Power Ice is generally the better option. It comes down to Overheat vs Hydro Pump.
The best move is likely Hydro Pump here. Rotom's worst matchup is undoubtedly Tyranitar, the perennial king of Gen 4 OU. It is easily Pursuited by Tyranitar, and running Overheat makes the matchup living hell. Specs Hydro Pump allows Rotom to 2HKO Tyranitar lacking Passho Berry without hazards up, and Specs also has a good chance to 2HKO Tyranitar after Stealth Rock, Spikes, and the comsumption of the Passho Berry. Hydro also has other uses such as obliterating the Scarf set's other problematic matchups like Grounds, which can usually take a Shadow Ball and recover / have the bulk to take several more (looking at you, Gliscor).
However, if your team has bad MU against Pokemon like Jirachi or Scizor, then Overheat can definitely be used. I'd still just use Hydro Pump though.
