I'm a big fan of games that offer me a lot of depth of customization to explore.
This puts good character-development driven RPGs and Survival/Craft/Base-building games at the top of my list. The more (non-gear based) character development the better or, absent that, a robust progression in terms of building/crafting satisfies as well.

I also love more complex strategy and simulation games and can get lost in colony management games so deeply that I forget to go to sleep.