It is straight forward that we want to save our favorites, legendary, the ones with the best IVs, and Pokémon from special events,
What is also important to know is that it may be a good idea to save Pokémon with specific IV and CPs for the great battle league GBL). Besides the great, ultra, and masters leagues, there are leagues such as the little cup that only allow Pokémon with a CP of 500 or less. In this league wynaut, alolan vulpix, ducklett, shelmet, and salandit are strong Pokémon. So we might miss out if we only keep all of the ones with a high CP.
A great site to find ideal CP of Pokémon you want to keep to evolve for the GBL is pokemon.gameinfo.io. We simply type in the CP of our Pokémon and we can estimate the ones we need to save for each league. In this example here, the CP of Bulbasaur it might too high. We can check the game to see what the CP it will have once evolved then redo it for Ivysaur on this website to see how close to 2500 it will be.
In the beginning, we all think that a perfect IV is the best for everything, but a maxed out Pokémon in the lower leagues is not ideal. A site to check out pvpoke.com to learn more abut it. They rank Pokémon to use for each league, help to create a team, and explain which attacks are ideal. We can also train and run simulations.
Then stadiumgaming.gg is a website that helps to find the best IV for each Pokemon depending on each league. They also explain why different IVs play a role. For example Umbreon’s best IV for the great league is 0 (attack), 15(defense), 15 (HP). The example below shows one that is 0,14,13 which is ranked 25th best.
Each GBL season may have different leagues and we often don’t have enough Pokémon space to save them all. But these sites can help us decide. Sometimes we get lucky as save the ones that are helpful and sometimes we don’t. In the end, the most important advice we can have is to remember to have fun!