Point Lookout
Broken Steel
The Pitt
Dead Money
Mothership Zeta
Operation Anchorage
Between the 2 games themselves. I found both really good, but i think i enjoyed Fallout 3 more. The reason behind that is i personally liked fighting in the ruins of D.C and the Metro tunnles and I found exploring the wastes to be more fun overall also.
Things i didnt like about New Vegas was the fact that most of the places to explore had no meaning or point at all.. You found a shack.. with no one in it but a book and some junk.. ok ... I also thought that there were way to many NCR bases in the game. Everywhere you go there is NCR, not much exploring to do TBH... I admit tho some places in NV were really cool to explore and i liked the game but i dunno i felt it could have been a little better, it seems like they crammed to much in too small of an area i guess.