Baldur's Gate II - Throne of Bhaal
Sadly it is necessary to underline that by ending, I mean the very end; things that happen after last boss goes down. Story&villain of the expansion as a whole were anticlimatic and dumb bullshit.
But what a long story BG 2 is! I'm sure you can make like 200 hours disappear in the game with ease. It took me almost 10 years to finish the game. Mainly due to story of expansion always just driving me away.
....Yet it makes an awesome game as a whole, and I never forgot about it. When story remains on-going and untold for a decade, it grows larger than life in your headspace by default. Made a very emotional and powerful thing to finally finish it some years back.
Very honorable mentions that, unlike ToB, shine bright entirely with their own merits:
The Last of Us
Walking Dead S1
More I think of Last of Us, greater value it has in my eyes. Awesome story and narrative in perfect marriage with awesome gameplay. In TLOU, story that gets told actually fits and compliments the gameplay you are exposed to. And other way around. This is a surprisingly rare thing.(Example, In SW:TOR your story is of a Jedi Sage who means to save his master from a grave ilness. Gameplay and what you actually - do- in the game revolves around being a mass murderer who kills few dozen people for irrelevant reasons. Story and actual game fit together like dollar bills to a slot machine.
Like TLOU, Walking Dead comes with awesome story and characters. Yet, it makes a game that sacrifices literally everything else on altar of narrative and story. Playing it is a powerful, awesome experience. Yet, there is barely a game in there to speak of. Comparign the two makes me think even more highly of TLOU. In terms of emotional attachment and great characters, it worked for me just about as well as Walking Dead did. Unlike WD; it actually comes with an awesome game to boot.