The Elder Scrolls 5: Skyrim (Special Edition). The reason: Mods. Lots of mods. Seriously, I have thousands of them from sites all over the Internet (be it from mod hosting hubs, forums, personal blogs and Tumblr before the big exodus to greener pastures) as well as a few personal ones whipped up to my specific tastes.
With enough rejiggering of mod load out, one can effectively recreate the game to give oneself a whole new experience every time.
I would pick The Sims 4 as my runner-up for the same reason, only I enjoy the base gameplay loops less and the mods are not quite as numero