I'm surprised Deoxys-S didn't actually make the candidates to choose from. In that case I am going for Arceus Water here, reason being it had a similar asset to Klefki fitting on a large number builds in both XY and ORAS. Being the crux and backbone of most balance teams, due to its ability to accommodate for role compression such as defog support, but its defensive capabilities to check key threats throughout the generation (Pdon, Ho-oh, kyogre pivot, ground arceus, mence) to name a few. The essentiality of having a support Arceus on the majority, if not all, balance builds, while fitting quite a decent portion of the bulky offense builds and its terms of being to fit more builds than other niche formes highlights it as the most dominant Support forme.
Going to stick with Arceus-Water for #9. A perennial presence on a plethora of archetypes, Waterceus was by far the best support forme of Arceus, indubitably outclassing other formes at that designated role.
Also, all my 3 noms have now been officially included in the rankings for good :)
I am still going for Arceus Water. For reasons outlined here. I don't think Lugia should make this spot, Lugia has really been on the decline on ORAS over the period due to metagame trends, and the Archetype it defined isn't a very workable playstyle in ORAS.