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.