That really depends on what you're using it for (single or double battle), what plate you want it to hold, and what other Pokémon are in your party. I don't battle competitively, so my opinion probably isn't the best but I'll give it to you anyway.

If you got it in the Dream World event, it should already know Recover, Judgement, Perish Song, and Hyper Beam. I would get rid of Perish Song and Recover. I'm not a big fan of Perish Song and a Max Potion or something might be more worth your turn than Recover. If you didn't get it in the DW event, it'll learn Hyper Beam at lv 80 and Judgement at 100. These attacks are really strong, and if you leave Arceus as Normal type it'll get a same type attack bonus for both of them, making them even stronger. I would also teach it Earthquake and Overheat, just for the pure power. Arceus is the strongest Pokémon and can learn almost any move, so you can really teach it whatever and it'll be fine.