Kerbal Space Program (with or without mods)
Minecraft (with or without mods)
Sim City 4000
Fallout: New Vegas (with mods)
TES4: Oblivion (with mods)
Dhampster, you have not commented. What would you take?
I was waiting for people to post before I shared.
Sid Meier's Pirates. I choose this because I never get tired of it.
Europa Universalis 3. Yeah, there's 4. But 3 has a built up selection of mods.
Morrowind. And I'd load up an external drive with mods to bring with, and it comes with the tools to create your own content. A lifetime of gaming just by itself.
Minecraft. A procedurally-generated lego set. What's not to love?
Mount and Blade: Warband. With mods, it is in the same category as Pirates: never get tired of replaying.