Europa Unuversalis IV - Multiple ways to rewrite history. Has plently of mods available for new experiences Euro Truck Simulator 2 - Comfy truck driving sandbox game Warframe - Alerts, clan, events, and grinding for mods and parts. STALKER series - free-roam elements and the a-life always makes something interesting happen. Lots of mods.
Any game with no actual objectives (Strategy and simulations) like The Sims, SimCity, Civilization and Euro Truck Simulator 2, for instance, are perfect examples of highly replayable games. Games where you try to get the highest scores (SHMUPs, pinball, arcade titles) are good too for replayability.
Max: "I think he just needs a hug, or a sharp blow to the head."