Theoretically, all the plates work well on Arceus as it learns coverage moves regardless of what plate it holds including moves such as Ice Beam, Thunderbolt, Flamethrower, and Earthquake.
Generally, the Electric Plate is commonly used due to Electric only being weak to Ground. Arceus makes up for this by having access to Surf, Ice Beam, and Energy Ball.
The Dragon and Spooky Plate is also abused since Dragon and Ghost are weak to themselves, meaning Arceus's Judgement already covers opposing Dragon and Ghost types, should you use their respective plates. Ice Beam, Dark Pulse, Shadow Ball, Focus Blast, and Flash Cannon can be used to punish Dragon Types, Ghost Types, Dark Types, and Fairies respectively.
Finally, out of the rest of the Plates, the Iron Plate is seldom used a bit more than the rest (excluding the Electric Plate) because of the sheer number of resistances Steel possesses. Toxic, Thunder Wave, Will-O-Wisp, Recover, Protect, and Cosmic Power all work very nicely with this Plate, as they increase Arceus's stalling power and longevity.
On a side note, the Earth Plate can be used for one of the most destructive Earthquakes in the game, if you needed Earthquake to murder anything. Hope I helped! :)
