Fallout 4 for me. I'm pretty sure it's blazed past Skyrim for most hours I've put into a game. It's all the possibilities, all the ways you can shape your game, that make it the best roleplaying game I've ever booted up. I'll play other games and while I'm doing that, they'll give me ideas for things I want to do in Fallout 4, which prompts me to jump back in and start a new game with a new theme or purpose. For example, I've played in a world where the Minutemen were going down a dark path of oppression and violence. In another, I created a lot of raider controlled settlements. In another, I expanded the Institute's presence above ground.
Portal 2 is up there, as well. It succeeded on SO MANY levels. Storytelling done right.