Reus

Reus is a fun god game.  You control four giants (ocean, mountain, forest, and swamp) that are trying to foster humanity by controlling the environment.  The game is fairly simple -- you can put animals, plants, and minerals down near towns, and you can upgrade each of those natural sources, and humans sometimes war with each other if they get too greedy.  However, there is a ton of intricacy because you need to choose what types of towns to develop, how you want them to develop, what natural sources would best help them develop, and how to upgrade your giants to provide those natural sources.  I spend a lot of time planning before even starting a game, and a lot of time paused when in game to figure out how I'll execute my plans.

If you're in the mood for something lighthearted, easy to get into, and challenging, I recommend Reus.