Either one or both works.
Mine is completing the Pokedex in the original Pokemon games. All you get is just Professor Oak giving you a wink and a small few second cut scene. And a congratulations text. Imagine spending all of your time then, getting all 151 and even 252 pokemon just for that? Yeah no thanks, I never completed the pokedex.
Going the Joja-Route in Stardew Valley. I say this mainly because, it is what you make of it. You forfeit being able to complete the Community Center by earning things, when you sign your soul away to Joja. What I would’ve liked is seeing Pierre go out of business completely. I just think that would add a route of depth in the game where you have to make ends meet through Joja because Pierre is permanently closed.
But, that doesn’t happen, he’ll still be in business despite his depression about Joja running things. Kindof ruins the whole concept of doing it for the achievements even.
As someone who has in fact completed both the original Gen 1 and the full Gen 2 Pokedex (including Mew and MissingNo.), I genuinely can’t imagine playing through a Pokemon game without at least completing the regional pokedex. Collecting the creatures is what I play those types of games for.
And the reward isn’t the little completion diploma Oak gives you to print out. It’s the self satisfaction that comes with finishing your goal. Like getting all the achievements in a game; I don’t get anything whatsoever for that, but I still like to do it. Because I’m a completionist.