Favorite moments thus far:
1) All of the Dunwich building. Very immersive zone. Read the one computer terminal with the program that transcribes voices; it captured the nuclear bombs falling 200 years ago.
2) Finding (what's left of) the White House. Find Pennsylvania Ave. in the Capitol Wasteland, and look for the iron-grate fencing surrounding a closed-off area. There... isn't much left.
3) Unleashing the "Chinese failsafe" option in Dr. Brant's Tranquility Lane simulation and dooming Brant to being locked in the simulation forever... all alone.
4) Charon "having a word" with his former employer in Underworld after you buy/steal/obtain Charon's contract.
5) Getting caught in between Talon Company mercenaries and Super Mutants as they battled over the remains of the U.S. Capitol.
6) Discovering that President John Henry Eden isn't quite what one expected him to be.
7) Signal Oscar Zulu. Both listening to the radio signal and discovering the source of the radio signal and finding out what happened to them made me sad, actually.
There are many, many more... but those jumped out in my mind.