I must admit Cookie Clicker got me engaged for a lot more time than I anticipated for an idle game, mainly because of how satisfying and rewarding the game can get once you have a enough prestige levels to ascend and then go back to the beginning to produce 100x faster than before.
By the way, the lore goes crazy on this one. I did not expect at all the grandmapocalypse event and its effects, the game turned way more interesting from that point on for me.

Pleasant interface, great atmospheric music (Steam version) made by one of the Minecraft composers and a great time overall. Definitely one of the best idle games out there.