Arceus's English name has had three different pronunciations. Before it was officially revealed, fans pronounced it as "AR-see-uss", as does Pokémon Battle Revolution. Then, a promotional trailer for Arceus and the Jewel of Life pronounced it as "Ar-SAY-us". Its official pronunciation was then revealed in a Pokémon.com mailbag as "ARK-ee-us". Its original pronunciation is still in popular usage.
It is ARK-ee-us