I personally can't decide if Poke, Ultra, Master, or Premier would best suit the legendaries I catch.
>Poke
Games: Hop's zacian/zamazenta and Gladion's silvally are kept in regular Poke balls. Plus pre-SunMoon pokemon like N's reshiram/zekrom. Gift pokemon like Cosmog are also still given in red Poke balls
Anime: Tobias' darkrai/latios, Nurse Joy's latias, Brandon's regi trio
>Ultra
Games: Anabel's latios/raikou/entei
Anime: Red's mewtwo/articuno/zapdos/moltres, Brendan/May's deoxys
>Master
Games: Team Rainbow Rocket's kyogre/groudon/dialga/palkia/xerneas/yveltal/zekrom/reshiram/mewtwo
>Premier
Anime: Gladion's silvally