Are you people actually looking at their stats? Some of you are way off base.
Appearance? Too subjective to pick one. There is no definitively "best-looking" Pokemon. As for power, no Pokemon in existence outmuscles Deoxys-A. Speed? Deoxys-S. Defense? Deoxys-D. HP? Giratina, one of only a few defensive legendaries.
If you bothered to look at the stats you'd see that Deoxys reigns over other all legendaries in terms of individual stats. Arceus, however, has the highest combined base stats, so it could go either way in a battle.