As a single stand-alone game, I would say Dragon Quest VII. The game exceeds 100 hours with no side quests, and unlike the Elder Scrolls series in which the games can be beaten in a few hours or less
Morrowind world record in 3:14.
Skyrim in 46.20 you can't do that in DQ VII. The story has to be followed, you can't just get gear, a few spells and levels and skip to the end.
Mass Effect was a trilogy spanning three games, but was kind of a mess.
The Diablo series, 1 through 3, All follow a single story line, but the story is pretty thin.
Dragon Quest 1-3 are stand alone games that are related to each other, same with 4-6 in the series.
But perhaps, the Witcher series is the largest. Each of the three games are huge stories and all three are connected with the ending in each game leading into the beginning of the next, so it is basically the story of Geralt of Rivia.