#bg:tob

What makes it special is that throughout 3 parts the plot is extremely personal. It is directly connected to you, your life, your soul, and your destiny.

BGI: Sarevok is your murderous half-bro who has had it in for you personally for a while. He also killed your father figure, so until you learn about Bhaal business it's a personal quest for revenge to find him. Even if you don’t know who he is you still end up foiling his plans making him hate you even more. Going back full circle to Candlekeep is amazing too since it gives you glimpses of your old life, the people you knew, and your relationship with Gorion who loved and protected you as if he was your blood. Super personal.

BGII: Irenicus tortures you and kidnaps your childhood friend. He also wants something from you which you need to figure out. Even if you decide to not give a damn about Imoen, you still have reasons to look for him (although admittedly aside from revenge on his torture and his hints on power, the reason becomes weaker). First part is you trying to rescue Imoen, get back at him, and find out what he wants. Second part is basically you succumbing to raw power of your blood, you fight for your life and sanity, and try to restore your and Imoen’s souls. You can’t get more personal than that.

TOB: Well now the Five want murder all remaining Children so you have no choice but to go and fight them. Plus you are shunned by everyone due to actions of your siblings. The real showdown for the throne of a dead god starts, or your way to get rid of the taint once and for all. In addition you learn that Sarevok hated Gorion and you even earlier than the first game implied, since you were literally a baby. Which is kinda awesome and develops stronger connections between all of you.

There are of course many things that could have been done better, quite a bit of plot holes and stuff. But the overarching plot is strong enough to make you fully invested in the story. I only wish TOB was an actual third game which was not cut so badly down to a mere expansion.