First of all, Arceus is the God. He can destroy them all if he wants to.
But if we are talking about a battle in the game, then I still think Arceus. He has amazing stats all around, but his ability helps him the most. He can just change into a bug type and eliminate the psychics with bug attacks. Genesect can't do much against a bug or dark type, so Arceus would eventually destroy it too. Deoxys might be a problem, depending on what form he is in.
I think Arceus, Deoxys, and Genesect are tied for coolest looking. The God, an alien, and a metal bug with a laser beam attached to its head. They're pretty cool.