It has to be the sudden and short-lived romance in between Acts 3 and 4. Hot af.
Joking aside, I don't think there were so many stand-out moments, which sounds bad, but it's because the game overall is very good from beginning to end. Maybe it was some battle, which took me by a surprise (of which there were many); like the blackpits one, first you have these lousy oil voidlings filling the area with oil, and then the fire ones come, you've got to be kidding me...and Alison, anyone?

I think the game is filled with many, many great small moments, that it's hard to pick one as a favourite, but I could agree with you on this one.