You always want to evolve the best Pokémon possible. “Best” being defined as having the highest stats (IV). The better the Pokémon you evolve, the higher the CP (Combat Power) and HP (Hit Points) it’ll have, and the better it’ll do in Gym and Raid Battles.
An update to the built-in appraisal system for Pokémon Go makes it easier than ever to see what you’re working with. Previously all you got was a phrase that would let you know where your Pokémon’s stats were. However, now you get a graph and a star rating. It breaks down the attack, defense, and HP of the selected Pokémon, as well as giving you a one, two, or three-star rating. This makes it easier than ever to know which Pokémon is worth evolving, and which you want to transfer to snag more candy.
It’s also easier to find your higher IV Pokémon using the search bar. By typing in
4*, you’ll see all of your 100% IV. The search terms also work with
3* so that you always evolve the Pokémon that will be most worth it. There are also a variety of other terms you can search for to isolate specific Pokémon, and you can find them all here.
Now, not everyone is into stats, not in baseball and not in Pokémon, but if you are, there are still third-party IV apps available to you, and they’ll make sure your every Pokémon you evolve is a monster. That way, you can evolve the 100% Bagon into Salamence and not the 82% one.
Note: Never use any app that asks for your Pokémon or Google login to work. You risk losing your credentials and potentially getting data stolen is never necessary. On top of that, you will likely be shadowbanned.