Skyrim. I know, it's been re-released a dozen times now, it's buggy as fuck, etc etc, but fuck me if it isn't an enjoyable game, even without mods.
Fallout New Vegas. It doesn't treat you like mr savior of the universe, you're a (un)lucky nobody caught in the middle of a power struggle. No essential NPCs, you can kill everyone you come across.
Age of Empires 2. Old as fuck, still enjoyable. Thank god the remaster lets you play with higher screen resolutions.