Most recently for me, Space Engineers. There was a long period where it was kinda eh, but since they added planets there's somewhat of a sense of progression involved in managing to get back into space, so I ended up putting in another 10-20 hours messing around there. It didn't really hold up in the sense of encouraging me to build mega-projects though, unlike Minecraft where I've made insane puzzle dungeons for friends to run through and things like that.
Also, oddly enough, I was more drawn in by the town-building stuff in Fallout 4 than the actual plot/shooter gameplay.