When I was younger, I spent literally thousands of hours (each) in three games. They were:
Civ 1 (DOS),
Civ 3 Complete, and
Railroad Tycoon 2 [now expanded into Railroad Tycoon 2 Platinum Edition, available on Steam (region-restricted), and GOG (ROW)].
I also really enjoyed and still enjoy:
Minecraft,
Sid Meier's Pirates! (the 2004 version),
Oregon Trail 4,
Chip's Challenge,
Baseball Mogul (old versions), and
Carmen Sandiego Games.
These days, in addition to those mentioned above, I must include
Arma 3 (missions: KOTH, gtaboncer's B.R.S.E., GrandElemental's Bounty Hunter (Malden), Pilgrimage, and the mod "PLAYERUNKNOWN’s Battle Royale by PLAYERUNKNOWN"),
Cities Skylines
Euro Truck Simulator 2, and
Baseball Mogul (new versions).
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Rocket League, though most of the fun is multiplayer.
Minecraft
Really any strategy game like the total war series, civilization, fallen enchantress, endless legend, etc.
Divinity Original Sin 1 and 2
Diablo 3, Torchlight, Titan Quest, anything in that vein
Audiosurf (as endless as your music collection is)
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Fallout: New Vegas
Dragon Age: Origins (for the gameplay)
Dragon Age: Inquisition (Personally, I replay it for the romances)
Dishonored
Binding of Isaac
Skyrim
Borderlands 2
Vampire: The Masquerade Bloodlines
Sid Meier's Civilization V
Don't Starve
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Go, better playing real opponent, but also traning AI possible
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Fallout 1/2. Every time I play them I discover something new. And the character creation has so much freedom that can totally alter the gameplay
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Slay the spire, singleplayer roguelike, in early access but well polished.
MTGA, multiplayer, still beta yet.
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There aren't many games I've finished and replayed many times without ever feeling bored. I can only think of two:
The replay value of Diablo 2 comes from the different classes and random loot you can find.
Lufia 2 on the other side is highly linear, but the story is so good that I couldn't get enough of it. And it has this highly challenging, randomly generated 99 Level dungeon that is optional, but has your characters all start at Lv.1 and without any equipment/spells. So this dungeon crawler part is basically a game of its own. It won't even let you keep your stats and 99% of the stuff you find, but the remaining 1% are the blue treasure chests. Their content you can keep for the main game and the stuff is really good. So it's always a joy when after countless battles you open a room and find a shiny blue chest in the middle, waiting for you.
Ah, good times.
edit: Oh, I just remembered a multiplayer game that also kept me going for 24k battles (1 battle usually takes around 5-8 minutes). World of Tanks. They have many different maps to keep it interesting and the fact that your team as well as the enemies are selected randomly makes every fight different.
That and the RNG that may have you miss shots on point blank or hit enemies with a one-in-a-million chance :)
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Especially if they can offer infinite replay value on single player
You can mention games whose replay factor is due to multiplayer as well, but please make it clear if it's the case
(replay factor = you can play it as many times you want without getting boring - even if you already finished the game)
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