We are trying to rank the most defining Pokémon across both XY and ORAS. To do this, one should examine either of two main aspects of a Pokémon: if offensive, its ability to exert team-building constraints, and if defensive, its ability to alleviate said constraints.
Deoxys-Attack ran two sets successfully in XY: Focus Sash lead on hyper-offensive teams and its notorious Life Orb set. Its Sash set was widely popular in the tournament scene thanks to its combination of being able to set up Stealth Rocks fast and Taunt, much like Deoxys-Speed, but with the added benefit of carrying with it massive offensive presence. In a metagame sans the Magic Bouncers, the lack of necessity in Skill Swap led Deoxys-Attack to be a staple candidate for consideration as Stealth Rocker for offensive teams. This is not to mention added versatility in its ability to learn Spikes.
In ORAS, with even further radicalizing stats across the board, the metagame shifted in equilibrium towards heavier offense. It goes without saying that Deoxys-Attack benefitted greatly from this. With a good speed tier and a Psycho Boost so raw in power even the specially defensive variant of the ubiquitous Pokémon in Primal Groudon risks a KO at reasonable HP, its coverage in Superpower, Ice Beam, and Knock Off caused huge team-building constraits for all teams created ever in the metagame. Its offensive utility is only further compounded by moves like Pursuit to trap Mega-Gengar and Extreme Speed for priority.
On the other hand, first of all, Arceus-Water was adjacent to being completely useless in XY. It was a jack of all trades but master of none in soft-checking a variety of Pokémon but to not much avail, with Kyogre, Palkia, and Ho-Oh coming to mind; all of which can break Arceus-Water with the correct moves or even just weather in place.
Obviously, Arceus-Water has seen a huge improvement regarding its footing in the metagame in ORAS but, really, this phenomenon is merely a reaction to how offensive the metagame had grown to become (the wave that Deoxys-Attack has ridden), thus increasing its viability as a defensive glue. But again, it suffers from trying to do too many things at the same time and, in terms of defensive utility, does not offer anything solid.
A Pokémon nominated further up the ranks in this thread is Lugia. Comparing these two Pokémon as defensive utilities, whereas Lugia can wall virtually anything and Toxic-stall or Whirlwind threats thanks to its over-powering Multiscale ability, Arceus-Water Defogs or checks things in similar fashion but the scope of threats neutered by this Pokémon and the efficacy of such facility is questionable with respect to sheer defensive utility.
To put it more succinctly, Lugia's defensive utility is notable because it forces all teams to run Stealth Rocks and, preferably, a Toxic-absorber, whereas Arceus-Water is a purely reactionary defensive glue.
Ask yourself this: when building in ORAS, how much do you need to prepare for Deoxys-Attack, and are the consequences grave if you fail to prepare for it or misplay against it? Next, ask yourself whether the defensive capabilities Arceus-Water provides outweigh this subjective metric.
In my opinion, the threat that Deoxys-Attack poses and thereby constraints on the team-building process far outweigh any of the subpar defensive capabilites Arceus-Water is able to provide. Its far lower ranking in the final draft of the viabity ranking is a testament to the idea I've attempted to convey here, in addition to Deoxys-Attack being leaps and bounds better throughout all of XY lead me to the conclusion of my nomination for Deoxys-Attack.
P.S. I hope that my literature helps sway some Deoxys-Attack voters!