Xcom Enemy Unknown... and then some
It's got all the joy of the first game with significantly more customization and replay value.
Ever since I beat Xcom EU on normal I have stuck with playing any Xcom games on Ironman on hard.
It's brutal but it's damn fun. The thing that wears this down in Xcom EU is the recycled maps, the lack of customization and the repeating mission types.
Xcom 2 spices that up significantly and makes it a lot more 'roguelike' to start a new game up and take your shot at saving the world all over again.