I would like to add to this discussion by saying that I am in 110% concurrence with Hack, Sweep, and perhaps others (I have not read every post here) regarding the ranking of Primal Kyogre. Primal Kyogre is, in my opinion, just as over-centralizing as Primal Groudon in ORAS but in an antithetical fashion; the two's abilities go hand-in-hand when concerning the viability of Pokémon in this tier in general. The reason that Primal Groudon is on 99% of teams can be largely attributed to the mere existence of Primal Kyogre, as it would otherwise be too overwhelming of a threat.
This is reflected in the changes I made to a team that I built in XY, which underwent major tweaks upon the arrival of ORAS. In XY, the team that I made consisted of the following: Deoxys-S, Groudon, Darkrai, Rayquaza, Xerneas, and Ekiller. My team-building process for this team reflects the over-centralization of Kyogre in XY as the team was built to abuse its prevalence. This was done by switching Groudon into Kyogre's move, letting it die as it leaves sun up on the field, then bringing in one of the four set-up sweepers to sweep. This is done whilst keeping Stealth Rocks on the field and wearing down the opponent's team before the sweeper is brought in. Credit for this tactic goes to alexwolf and Transcendent God Champion who came up with the idea of beating Kyogre by utilizing Groudon's sun in BW, and I believe
orch can attest to the effectiveness of this team and tactic through the numerous games we have played in preparation for the currently on-going XY tournament.
However, the introduction of Desolate Land and Primordial Sea altered the landscape of the metagame entirely. Since the Sun brought by Drought fails to replace the Rain brought by Primordial Sea, I had to give my Groudon a Red Orb to turn the tables on Primal Kyogre. The fact that this team subsequently became the principle outline for standard hyper-offense proves the fact that while Primal Kyogre is a threat necessary to be reckoned with, the reality of Primal Groudon running rampant alongside it means that, one, Primal Kyogre has a difficult time doing its job, which leads to two, that it faces competition from Primal Groudon in checking threats. Due to the overvaluation of Primal Kyogre in ORAS by the Ubers community, there may be those who disagree with Primal Kyogre's ranking, but this post serves as a bolster to the drop in this Pokémon's ranking.