Arceus without a doubt. In smogon formats, Arceus is in the ubers tier, which is the second highest tier and home of most of the boxart legendaries and that is Arceus in all of its formes. Silvally on the other hand, you can count on one hand which formes are in a tier, and that tier is pu, the lowest tier. Most of its formes are even untiered
They cannot be compared in vgc as Arceus is not allowed in that format but even then, Arceus has a massive bst of 720 compared to Silvally's 570 and Arceus has a very deep movepool
