Even without taking the legs into account, I'd say Arceus is still more like Dialga. They all share the same face, but Arceus has spikes on its neck and various bump-like protrusions on its body, things Dialga has. I also think Arceus' tail looks slightly more like Dialga's.
Palkia is, overall, more of a streamlined design, something Arceus only has on its head. --- If life gives you lemons, sell 'em to Tom Nook. Black 2 FC: 0132-9980-0476, 3DS FC: 5284-2520-2972 (Pokemon IGN: Aki)