[trickroom]

I would suggest running Arceus-ghost over normal Arceus because then you are immune to the ubiquitous ExtremeSpeed on other Normal Arceus. Additionally, there isn't really a good reason to ever run Return over Extremespeed on an Arceus. The extra-oomph isn't worth the lack of +2 priority that also gets STAB. Your set will get destroyed by either Giratina (normal forme almost ALWAYS carries Toxic and it isn't unheard of on Origin forme either) and is out-sped by both Mewtwo's and Darkrai's and destroyed by both Aura Sphere and Focus Blast, respectively.

Arceus-ghost is weak to Shadow Claw everything Extremekiller uses though while Normal Arceus isn't weak to anything save Brick Break. The reason to use Return over ExtremeSpeed is because it has way more PP, useful for a wall and hits stronger. Also Wall Arceus doesn't need that priority since it will outspeed standard Ho-Oh anyway.

Giratina normal forme doesn't carry Toxic usually. It uses Will-O-Wisp almost ALWAYS all the time for stopping physical sweepers easily, not Toxic. Toxic is used on Lugia more. Ghost Arceus is outsped by both Mewtwo and Darkrai anyway. Also Mewtwo can smack Ghost Arceus with Psystrike or sun-boosted Fire Blast. Darkrai can just pummel you with 100% accurate STAB Dark Pulse, that's worse than against Defensive Normal Arceus lol.

As to your question.
A good partner for ghost Arceus that is often overlooked is Terrakion. The Choice Scarf variant can OHKO all of Ghost Arceus' checks like Mewtwo, Blissey/Chansey, Darkrai, Reshiram etc.

I will like to see how Scarf Terrakion can OHKO Mewtwo outside of a critical hit. Chansey has a chance to survive Jolly Close Combat at full health.