Focus
Ubers Tester Extraordinaire
Max speed and max HP is the norm for Calm Mind Arceus, because that makes you speed tie with other Arceus, or even outspeed them. Arceus's base stats are high enough that you don't have to worry too much about those few extra EVs being better spent elsewhere. Investing the rest in HP gives Arceus nice bulk while it can CM and Recover off damage, but maxing out Special Attack works as well if you are aiming for a quick sweep rather than relying on tanking hits. Either way works, I guess.