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Check out The Walking Dead by Telltale Games. The game's more of a movie with you making choices but I've never been more emotionally invested in a game in my life and it wasn't even that long.

Looking forward to playing Season 2 soon.

Pokemon Red: I remember getting the game for the GB Color back in the day - I almost never put the game down. Even today I still enjoy plugging in the ROM and taking it for a spin, whuppin' the gym leaders and the Elite Four with the first 150. 

Skyrim: Typical Bethesda - giving me too much to do.

World of Tanks: Good on Wargaming for making this game because combines two things I like to do - shooting things in giant armored behemoths and giving it a RPG feel with the research trees and XP gimmick. Plus, it's free to play (you might throw in a few bucks down the line)

Killing Floor: I remember when this used to be a mod for the Unreal engine. When it came out as a stand-alone game I was back to grinding and it brought back a lot of good memories. Steam says I have a 120+ hours on the game.

Mount and Blade: Warband : This is probably by far the #1 time-consuming game I've played. I'm a perfection nut so I've kept restarting my game to keep trying a new build (end up doing the same thing). Almost 250 hours logged. No regrets.

If I feel the game would be tedious through a second-playthrough (Fallout/Elder Scrolls kind of game) I would be inclined to cheat to progress a little quicker. But of course, it depends on the game - I'll be essentially playing Borderlands 2 six to eight times with the different characters and second playthrough and then that's not counting the DLCs.