>>54575450Knowledge of the current meta and just experience. Like right now, it's pretty much all just the general Good Stuff meta, with people trying out various other mons to see what fits the team. But typically in the current meta there's like 3 teams going around primarily.
>Good Stuff: Ogerpon, Flutter, Iron Hands, Lando-T, Thundurus/Roaring Moon/Rillaboom and Chien Pao/Urshifu>Trick Room: Bloodmoon Ursaluna/Ursaluna, Cresselia, Iron Hands, Flutter, Amoonguss, Heatran/Torkoal>Clang Soul: Clefairy, Kommo-o, Ogerpon, Sinistcha/Rillaboom, Iron Hands, Lando-TFamiliarize yourself with the core of Iron Hands/Flutter/Lando-T because they're the most common pokémon in the format and teams are effectively built around that core rather than slot them into an already existing team.
The issue being that we're still not certain what Reg E is going to look like, so it's very much the wild west out there right now.