Welcome to the LC viability ranking project. In this project, we will "tier" every Pokemon based on usefulness. Broader metagame discussion should be held in the metagame discussion thread. This is a place to talk about the rise and fall of individual Pokemon instead.
The viability rankings sort each Pokemon in the tier based on their viability and usefulness in the format. You would like to read this once before making a team so to get a grasp of what Pokemon are tiered the highest and what not.
In every tier's subforum, you probably will have come across a thread known as the Viability Rankings. The Viability Rankings categorize each tier's viable Pokémon into six categories: S, A, B, C, D, and E, with the most influential Pokémon residing in S rank and the most niche Pokémon in E rank. Each S rank Pokémon is a threat every team must prepare for and will also find its place on many successful teams.
Some of the most viable Pokemon in Gen 8 LC are:
Vullaby is the most viable Pokemon in Gen 8 LC, as it is ranked at S rank with a surprising usage of about 71% which already shows how much threatening is it and how much teams prepare for it. In fact, it's obvious from the fact that every viable team has a Flying resist dedicated to Vullaby. For example, Defensive Onix and Pawniard have a high usage of 39% and 38% respectively. Vullaby was even suspect tested some months ago, however due to the majority of votes not being enough for to ban it, it stayed unbanned.
Mienfoo is a great pivot and Regenerator abuser, with being a good offensive Fighting-type. It is ranked A+ in the VR, with a usage of 46% in the tier which is the second highest. Teams always have to keep their Vullaby check to Mienfoo, as great coverage granted through moves such as U-Turn, Knock Off and good moves such as High Jump Kick and Fake Out make it good offensively.
Onix is one of the best Stealth Rocks setters as well as the premier Vullaby counter because of its bulk, faster speed tier and a good ability in Sturdy. Its Rock-type STAB allows it to handle Vullaby comfortably, and can be a great Flying-resist for any team. As such, it's also ranked A+, with an usage of 38%.
These are the top 3 Pokemon, as ranked by the usage stats and viability rankings. You should read the VR post which was linked above, as well as the usage stats once so that you can know the tier pretty well, before you make a team.
Hope this helps!