Rotom fills in an important niche as a lead-killer, especially Heat Rotom, which is resistant to both U-Turn and Volt-Switch, the 2 common attacks that leads use to deal damage and flee. Here's my best anti-lead moveset:
EVs: 252 HP, 56 Sp Atk, 100 Def, 100 Sp Def
-Ominous Wind / Shadow Ball / Sunny Day / Rain Dance / Taunt / Reflect / Light Screen
-Charge Beam / Thunderbolt / Sunny Day / Rain Dance / Taunt / Reflect / Light Screen
I like Snatch, for its ability to steal all of the opponent's set-up moves, like hazards, screens, and stat-ups. Substitute is a good thing to set up if you avoid the initial damage, and with Heat Rotom's unique set of resistances, it could easily keep up a Substitute for multiple turns. The last 2 moves have a lot of options. Either you can try for all out offense (Shadow Ball and Thunderbolt), stat-up attacks for times when you have a Substitute up (Ominous Wind and Charge Beam), or try to set up effects of your own. I wouldn't really expect a Heat Rotom to set up Rain Dance, but it's definitely an option. Volt Switch can also be thrown in, as it's helpful for an anti-lead lead role.
If you're creative, don't copy and paste this into your signature (Started by 3.14kachu).