Cookie Clicker
A simple looking idle game with way more than I expected.
At first, the game looked dumb to me, I thought it was just "click a button, see a number go up". But after playing for almost 300 hours, it's much more than just that.
What surprised me most is how much the game keeps adding. You start by buying a few buildings and upgrades, but eventually you're dealing with prestige, achievements, golden cookies, minigames, different upgrade combinations and much more.
I have 573 of the 637 achievements there are to unlock. They were fun to unlock, but some were very tedious, an example is "Liquid assets", where you must reach $10 million of profit from the stock market minigame.
The game is pretty funny, I love the little quotes that it shows you like: "News: local guru claims "there's a little bit of ourselves in everyone", under investigation for alleged cannibalism."
After 300 hours, I feel like the progression has slowed down a bit. It might be because I'm trying to not prestiege/ascend too much for some achievements.
I'm not playing it as much as I used to, but I sometimes come back to it.
Overall, I recommend Cookie Clicker if you like idle or incremental games, achievement hunting, or just want a cheap game you can keep coming back to.