Kingudora
My favourite
That's a good question.What I'm wondering is where does the guitar come fromfor Overdrive, since Toxitricity doesn't seem to have anything like one on its body. Perhaps it creates one made out of pure electricity, i.e. a truly electric guitar? Kind of like how when Lucario uses Bone Rush in the anime it creates a bone construct out of its own magical energies.
Also, I'm thinking that Overdrive might finally be the clue for how you change Toxtricity's form. I believe that it goes into Amped form after using Overdrive, much like how Meloetta changes forms after using Relic Song.