Im not a stickler for those sort of things, I mainly just want them to complete the pokedex.
Every few years (usually when the next Pokemon game comes out) I get the urge to "Catch'em All". I'll start playing one of the Pokemon games (I've own one from nearly every Gen for my collection, but I haven't played many of them) beating it, getting most of the Pokemin in that game, and then usually burning out on the task of trying to get them all because of some annoying legendary or other rare pokemon.
Then a year or two passes and I get the urge to go back again, haha. Most recently I was playing through Leaf Green, and I was doing great up until Champion Road where I was under leveled and didn't want to slog through the area.