Tier 1: 5/5 Rating
Pokémon in Tier 1 are meta-defining. In no uncertain terms, these are the threats to look out for. If you’re not ready to beat these Pokémon, you’re probably going to lose to them. Key examples are Medicham and Azumarill in Great League, Giratina in Ultra League (both of them), and Dialga in Master League. While they may not always be the best picks every time due to the state of the metagame, thanks to their general power they’re absolutely worth looking out for.
Tier 2: 4 to 4.5 Rating
Tier 2 Pokémon aren’t necessarily significantly less powerful than those in Tier 1. They’re not meta-defining, necessarily, but they’re objectively good. Maybe they just don’t fill a niche quite as well as another Pokémon (for example, Steelix doesn’t fill the “steel tank” role quite as well as Bastiodon and Probopass). Maybe you’re not so scared of them that you need to hedge against them when you build your teams. But, for example, if you need an Electric-type Pokémon, you know that you can’t go wrong with just slapping Lanturn into your 6.
Tier 3: 3 to 3.5 Rating
Tier 3 Pokémon, on a whole, have...issues. They can still be very strong in certain scenarios, but more importantly they demand those scenarios to be strong. In Great League, Haunter destroys Medicham, and Magnezone consistently beats Skarmory. But on the whole they’re not that fantastic. Bring them not for their power, but for the specific utility they have that your team needs.
Tier 4: 2 to 2.5 Rating
Down in T4, we’re starting to get into stuff that’s just not great overall. While they often will see play in Silph League tournaments thanks to the limited metagames, they don’t match up too well to general top threats thanks to lacking stat distributions, or mediocre moves. They also don’t fill the “counter a relevant threat” niche that T3 Pokémon do.
Tier 5: 1 to 1.5 Rating
The Pokémon here are...not good. For any Tier 5 Pokémon, you can probably find something that fills a niche significantly better, or just has a more relevant niche.
Not on the Tier List: 0 Rating
Pokémon rated 0/5, while they may not technically be completely useless, should definitely be avoided. Deoxys-Attack, for example, will most likely faint before getting a single charged move off. You can use them if you want to show off how cool you are by not following the meta, but you’re better off looking elsewhere if you like winning.
What Do I Do with This Information?
As nice as it would be, you can’t just arbitrarily choose 6 Tier 1 Pokémon and expect to be victorious. While, on average, a Tier 1 Pokémon will be stronger than one in Tier 3, that doesn’t mean the Tier 3 one won’t fill a role in your team better. Wigglytuff, despite being T3 in Great League, can absolutely tear up various T1 Pokémon such as Altaria, Medicham, and Sableye.
That said, do not just look at the tier of each Pokémon. Like with any tier list, each entry has a significant amount of nuance to it, so your best bet is to actually look at the writeup for each Pokémon to get a better sense of how they work!