Fallout 4. The amount of world exploration and itty bitty stuffs almost makes me lost myself in exploration, even though the story can be really short depending how you progress the content. On my first playthrough, I clocked at ~90hrs of play time and only just passed the 1/4 of story progression just because I sucked in sidequest and exploration.

Never thought I enjoyed the base building and assisting settlements aspects, Bethesda did great job on Visual storytelling speaking as Interplay/Obsidian Fallout fan.

Another case is STALKER Anomaly mod which can gives you theoritical endless playtime as long as you creative to build your own CYOA Stalker story. Though I don’t recommend Anomaly if you’re looking for the STALKER lore (as they’re fan project) and should be treated as post-vanilla playthrough.