In my ancient days I would only have ever used Heat Rotom—'cause fire cool, what can I say B)—but nowadays I'd have to say Rotom's regular form is my favorite. It's just a lot cuter than the others :>
Out of the appliance ones, though, I'd say Wash Rotom, 'cause of that cute grin. Next would be Mow, then Heat, then Frost, then Fan.
Out of Bike Rotom, Drone Rotom, Smartphone Rotom, RotomDex, and Rotomi, my favorite's gotta be Smartphone Rotom 'cause of Raihan's synergy with his. Sleeping Rotomi is also very cute, though :> Drone rotom is third place, then RotomDex (just 'cause it's annoying in the anime) and then Bike Rotom, 'cause it's... just a bike.