It isn't surprising that multiplayer games offer endless hours of gameplay. I'm going to list gaming series that are not strictly an online experience and have high replayability.
Dark Souls series: Finishing one of them takes around 50 hours. But certainly the average player will not put aside these titles before investing at least +200 hours, Me personally invest more than 300 hours with Dark souls and is highly likely I'll replay it someday. Everyone return to face the challenges of a Souls game.
Planescape Torment: A tricky recommendation, since this title is a heavily story driven one and I dare to say has the best history ever told . You incarnate a amnesiac guy with no name struggling to discover his hideous past. You know a unique set of companions that will be influenced by your moral choices, and everytime I replay it, I notice something new didn't saw in previous playthroughs.
Fallout 2: The good Ol' fallout. When Black Isle Studio was the pinnacle of rpg videogame developing. You start as a chosen tribal to accomplish a simple mission: retrieve a G.E.C.K. A technology that terraforms a barren wasteland in a suitable enviroment for thriving life. But as you can expect, is not easy task to find something so advanced in a decadent, radiated and salvaged Mojave Desert. Accompany your character in an unforgettable experience discovering the last bastions of humanity and Monty Python references! Fallout has tons of Sidequest ad you can make ur own path and approach to solve them, and there are only two g.e.c.k.s in the game, It's up to you which path should take to retrieve it.
Fallout New Vegas: Unlike the retarded melting pot that was Fallout 3. New Vegas keeps true to the saga's origins and delivers a great experience somewhat crippled by a shoddy engine. You are a courier incidentally put in the middle of a geopolitical conflict in Mojave between New California Republic, Caesar's Legion Authoritarian regime and Mr. House Technocracy's aspiration. You can pick sides with one of the major factions and your actions will affect your global reputation.
Patched Jagged Alliance 2: Call of Duty?Medal of Honor? Rainbow Six? Counter Strike? Hah! Children's games. The true hardcore ones play the best squad tactics ever made. And was by a canadian developer studio! Thanks Canada!. Anyway, you have only a mission: kill queen deidranna, the Arulco Tyrant. You have plenty of liberty to approach this mission and hire the mercenaries that suit your playstyle best. Full Assault? Covered. One Man's army Night Ops? Covered. Sniper's camping? Check. Stab and hide? Also check. You can even finish the mission in less than 30 minutes, or spend weeks seizing control over arulco earning money and gearing up ur mercs. Never grow tired of playing it.