Cookie Clicker

Cookie Clicker started in 2013 as a joke about clicking a cookie and became the reference implementation for the entire incremental genre. Orteil kept building it, and the version that exists now has an ascension system, hundreds of upgrades, minigames attached to individual buildings, and a late game that requires actual planning.

It is worth playing partly because it is good and partly because it is the source, most of the vocabulary the genre uses, prestige loops included, is here in its original form. The browser version is free and complete; the Steam release adds music, cloud saves and Steam achievements.