Definitely minecraft. I've spent thousand of hours on this game. I can just play singleplayer to chill for some time or go on some multiplayer server and build something huge. The opportunities in this game are just infinite and that's what I love about it. And it's one of the easiest games to pick up again after not playing for a while. Doesn't matter how much I'm sucked into the story of some other game, I keep going back to good old minecraft.
The best thing about Minecraft is that, even once you think you've done everything there is to do, there's always more. Tired of the vanilla game? explore mods! In particular, the FTB modpacks are awesome and provide many many many times the content of the vanilla game.
There are lots of great servers, you don't even have to play with your buddies to have fun on most servers because the minecraft community is one of the nicest I've ever seen. Most servers are happy to welcome new members and will also help you if you want to start a big project.