>>35024399GSC comes first, it's also a loose sequel to the RGB and Yellow arcs the main characters appear and stuff from those arcs get mentioned a bunch but you can probably just jump right into GSC if the only thing you're reading for is the Johto trio. FRLG arc has Silver in it and is very important to his character, it also has the characters from the RGB and Yellow arcs in case that might persuade you to read those first. The Emerald arc is a bit of a sequel to the FRLG arc and has the entire Johto trio in it, they don't really get much, or any, character development in that arc and it's also infamous for being everyone's least favorite but the ending is kinda cool imo. The characters from the RS arc appear in Emerald but it's not necessary to read that in order to understand whats going on. HGSS takes place after that.
If you just want Johto: GSC > HGSS
If you want everything with the Johto trio: GSC > FRLG > Emerald > HGSS
Alternatively you can just read the whole thing since the early arcs are more connected and reference each other a lot.
