I actually went on a quest to find the one idle game worth playing!
Uhhh... yeah we already played the good ones years ago... move on.
This isn't totally true, there's still good idle games coming out if you can find them. Two I've played recently are:
Factory Idle where you create increasingly complex factories using conveyor belt and assembly machines to produce components that you then sell (or research for new stuff). It's like if factorio was an idle game (and without aliens.) more or less, and a really good take on the whole thing. I'll admit I didn't actually get around to the total endgame (for various reasons including a bit of boredom.) but for the days that I did play this this was one of the best idle games I've played. !!Warning!! you gota like it to, well, like it. If you don't want to play factorio tetris you're not going to enjoy this game. If you do like factorio tetris, you'll love this game.
Spaceplan is pretty short, you can finish it in a single session of playing it every once in a while in the background. It's a bit more a traditional idler then factory idle, but the interesting things that happen and that you can do that don't wear themselves out because of the short runtime make it quite an entertaining ride.
For browser games in general, I'm still playing
Wilds.io. One of those multiplayer pvp io games, it had a thread on bay12 a while ago. In this one you control a dude with an axe axing other people in one of two "teams" in a arena littered with rocks and lethal sand vortexes. Despite the pay to win nature of being a patron to the game and the devs mercenary approach to the games development (which with it's almost nonexistent patron base means there's effectively no development) the like, surprising depth to the combat and just the fun of it keeps me coming back way way way longer then I've done with any other IO game.