What is the best move set for Arceus?

Answer 1

Have it hold either a Dread Plate or Draco Plate

Draco Plate (Special): Draco Meteor, Judgment, Earth Power and Hyper Beam (Max Speed + Special Attack)

Draco Plate (Special v2): Calm Mind, Draco Meteor/Judgment (choice), [Earth Power, Flamethrower/Overheat] or [Ice Beam, Surf] (choice) (Max Specials, Calm Mind restores lost Special Attack From Draco Meteor and Overheat, too bad no Nasty Plot...)

Draco Plate (Physical): Swords Dance Extremespeed if someone Swords Danced already and used Baton Pass, probably Ninjask), Outrage, Earthquake, Flamethrower/Fire Blast/Overheat (your choice) (Max Defenses and/or HP + Special Attack)

Dread Plate Move Set: Judgement, ExtremeSpeed, X-Scissor and Dragon Claw [Max Attack + Speed (Optional)]

Life Orb/Leftovers/whatever: Swords Dance, Extremespeed, Earthquake, Shadow Claw (Max Attack)

Spooky Plate: Shadow Claw, Brick Break, Recover, Refresh/Substitute/Swords Dance (If no one to Baton Pass it) (Max Attack + Speed)

Splash/Zap Plate: Judgment(Splash)/Surf(Zap), Thunder, Ice Beam/Blizzard, Rain Dance/other (If you switched out ROM someone like Kyogre) (Max Speed + Special Attack)

Wall (Leftovers): Substitute, Toxic, Will o Wisp (Anything immune to Poison), Earthquake (Heatran resists both) (Max Defenses + HP, Watch for flying types with Nature Cure, bring something with Stone Edge to deal with that or Something...)

The last guy had some great suggestions; however, most of the sets that were recommended are only useful if you can EV train Arceus. Since the Event Item, the Azure Flute, has yet to (may be never at this point) been released by Nintendo, the only official and legit Acreus is from TRU Acreus distributed at Toys'R'Us last year (or any of the movie related tie-in events from other regions). Because this Event Acreus is distributed at level 100, it CAN NOT be EV trained though battles. Only though Vitamins can be used, thus giving it a EV restriction of 100 of 5 Stats and 10 on 1 stat (example: hp100, atk100, def10, SpAtk100, SpDef100, Spe100).

It also has signature moves from all of the Legendary Dragon Trio (Spacial Rend, Roar of Time, and Shadow Force) that Arceus otherwise can not learn. It is recommended to change its movesets, otherwise it will be walled by a Defensive Steel Type such as Forrestress.

Still, even with its EV restriction, Event Acreus has some useful builds.