There's tons of them.
Meeting Caesar.
Meeting House.
Listening to Raul's backstory.
Hearing out Ulyssess.
The whole Dead Money experience.
Same, In the base game I'd say meeting House and Caesar was great because they're both highly intelligent, but I think the Legion was a poor faction because so much potential content was cut due to time. Raul was a very fleshed out character, he's my number two favourite, Arcade coming first. Ulysses is indeed another thought provoking character, and I prefer helping keep him alive, and Dead Money is a work of art.
Reading Randall Clarke's journals.
See, I've seen this a lot, but while it's a good story, I never found it 'memorable' to me.