>>51614394Gen 2. Stadium 2 contextualized the entire game as a hardcore single player RPG, much the same way battle facilities would do later but I'd argue in a lot more satisfying way. It's barely a spinoff in that sense, because it ties so strongly to the main game.
Colosseum and >XD were cool too, but by the time Battle Revolution was out we already had "hardcore" postgame in the main games themselves, so it didn't feel as important.
I'm someone who can't stand Mystery Dungeon or Rangers or any of that crap though, I'm very mainline oriented so consider that.