Welcome to the Ruins of Alph Version of the ORAS UU Viability Ranking thread! In this thread, we as a community will be creating a valuable resource with the intent of helping people who are interested in learning how to play this metagame. With that in mind, the main goal is to organize all of the usable Pokemon in this tier into "tiers", from best to worst, with no differentiation between offensive and defensive threats. Everyone with experience is encouraged to comment, and every post will be taken into account when deciding possible rank changes.
The S tier contains a select few metagame-defining Pokemon, the best of the best, and the A tiers reflect on the Pokemon which are extremely effective but flawed in some aspects. The B tier is slightly more expansive, with the B+ tier representing Pokemon that are fairly common and threatening but don't quite make the cut for A-, while the B- tier represents Pokemon which fill minor, but relevant, niches. Below the B tier, the generic C tier displays the rest of the Pokemon that are usable in UU, but don't have a niche that is appreciated by most UU teams. Finally, the D rank will only include Pokemon that are tiered in UU by usage, but have no usability in the metagame.
All Pokemon are individually sorted within their rank by viability. This is quite subjective, and it's fine for users to make suggestions regarding each Pokemon's placement within those ranks in their posts, but it's usually better if they don't become the sole focus of the discussion.