>>44012663Pokemon are ranked primarily based on how often they are used, with the idea being that Pokemon that are good and work well consistently will naturally see more use by players trying to win. This is why the top tier is called OU, which stands for Overused.
Usually if a Pokemon gets used a lot, others have to make way for it. Like you would never see Flygon in a metagame where Garchomp exists. That's why the lower usage tiers exist, like UU (Underused) so that there is still a place where these not-quite-as-good Pokemon can be used. There are a few usages tiers below UU, as well.
Aside from usage, there are also banlists. G-Darmanitan was in OU for example, it was getting used a lot because it is insanely strong. Ultimately it was decided that the whole game basically revolved around who used their G-Darm better, so it was banned from use in OU. This is the only subjective part.
The "viability rankings being discussed in this thread are just a general, opinion based ranking / guideline of how good certain Pokemon are in the current metagame. It's totally unrelated to tiering.
