As in: you might take a break of a couple of months/years each time, but you know you will always come back to it eventually, even if you've 'completed' it already.

For me:
Bio Menace - first ever pc game, so incredibly nostalgic, and I still contend that it's a very good game regardless.
Island of Dr Brain - again, extremely nostalgic.
Warcraft 3
Dwarf Fortress
Catacylsm - DDA
Mount and Blade
Arkham City
Transport Tycoon Deluxe
Roller Coaster Tycoon 2
Caesar 3 / Pharaoh - my happy place
Civilization 1,3,4 or 5
GI Joe (nes) - for some weird reason I just adore this title
Lion King

Or perhaps it doesn't apply to you, and you feel no urge to replay anything you feel you've completed already?

The games I repeatedly return too :

Thief 1&2
Deus Ex (only the first one)
MediEvil 1&2
DOOM 1,2 and Final DOOM
Duke 3D
Blood
Quake 1&2
Civ 2
Monkey Island series

There is a lot, but i'd say The Witcher 1,2,3 and Mass Effect 1,2,3, those are probably my personal favorite so i will never have enough playing them :)

Going by what has a more or less permanent residency on/hardly ever gets uninstalled from the hard disk:

Heroes of Might and Magic 2
Heroes of Might and Magic 3
Jagged Alliance 2
Battle Brothers
X-Com: UFO Defense
Master of Orion (I)
Master of Orion II
Gothic
Gothic II
Planescape: Torment
Vampire: The Masquerade - Bloodlines
Deus Ex
Mech Commander
Mech Commander 2
MissionForce: CyberStorm
CyberStorm 2: Corporate Wars
Anno 1602
Quake III: Arena
System Shock 2
Thief
Thief II
Diablo
Diablo 2
Ultra (Tempest clone)
Crysis

Can't believe I forgot to include these! especially because of v1.13 and openxcom

Top of my head...

Any of the Wolfenstein games starting from RTCW
The Witcher series
Dragon Age: Origins
Neverwinter Nights
The Mass Effect trilogy
League of Legends

* UT99

Almost everyone in my family and friends owns it and still plays it.

* Dungeon Crawl Stone Soup

My most played SP game of all times. I'm rocking this one in the grave too!

* The King of Fighters 2002 Unlimited Match

My kids and cousins talk too much shit so they gots to catch these treasured hands.

Freelancer
Terraria
Rimworld
Factorio
Dwarf Fortress
Cataclysm: Dark Days Ahead
Distant Worlds (2?)
Beyond Good & Evil
Grim Dawn
Fallout 2
EV Nova
Dungeon Siege 2
Drox Operative (2?)
Trancendence
Tales of Maj'Eyal
Sacred gold

Any of the following franchises
Diablo
Descent
MechWarrior
Just Cause
Project Cars or eq.
Half Life
Portal

Plenty of games.

Interestingly enough, none of the ones listed so far in this thread (though I may try Terraria someday).

Candy Crush... because I'm too stingy to spend money buying helpful boosters for an online-only game.

HoMM 3
Civ, probably V.
FF: Tactics
Fallout: NV
Fallout 2
Disciples
AoW 3/Planetfall
Cyberpunk (Primarily because it's what we got right now.)
Shadowrun: Dragonfall
Skyrim, mostly modded to add all the things the base game got wrong.
Castlevania PoR/SotN/OoE/Rondo
Mass Effect 1
Star Control 2
Colonization
Alpha Centauri
Medieval 2 (Both base game and the DaC mod)
MoO 2
Terraria
GTA 4
KoF
SF Alpha series
House of the Dead
Path of Exile/Grim Dawn/Diablo

Solitaire, Chess, 麻雀 puzzle games and Bejeweled. Solitaire is my number one when I can't decide what to play next, when I just want to listen to something in the background and keep myself entertained thinking about things, it has been and will remain to be my game of choice to turn to. Bejeweled I have played on and off with very long breaks in between much like Chess.