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.
I do challenges for the fun of doing them, not for the reward.
Dark Souls at level 1 is absolutely worth doing if you’re replaying the game. It’s a lot of fun, and much easier than you’d think during your first time playthrough.
Surprisingly enough, playing Getting Over It 50 times over for the achievement is one of the most fun experiences I’ve ever had. The skill curve is just awesome to experience, my time dropped from 18h all the way down to 10 minutes.
Challenge not worth doing : all achievements on Rayman Legends. The game itself is amazing, but to get the last achievement you need to play daily procedural levels for MONTHS unless you’re a god at the game (and I’m pretty sure the top times are always cheated anyway, so you’re never getting the highest point reward anytime ever)