For me it would be Fallout 3 because:
Storyline
Fallout 3's storyline was very good it really made it feel like an RPG because you played the start of the game as a baby and then child knowing your dad then the main game starts when you need to hunt him down and then he dies and you must not let his project run out.
While NV's storyline is about a random man who gets shot and you try to figure out who you are and then hunt him down and once you do you go with one faction and take over Vegas....pretty random.
Quests
Fallout 3 had main quests and some side quests not that much to do..this i dont like.
NV has lots of main quests,side quests and faction quests which is good
Setting
3's setting was awesome it was the 1st Falllout game i played and its what made fallout my best game serious becuase of the big,rundown,sad and depressing setting it had with ruins everywhere, people die, fire ect
NV's setting was pretty clean it had the nice big clean and secure Strip, clean goodsprings ect. To me its like a cowboy setting with robots and lasers
Now for me the best game would be:
One like F3 with a big open depressing setting with lots of characters each with lots of quests along with the main quest and after you complete story mode you can still play and build up your character.
With NV's ADS and guns and attachments then it would be awesome
Now give me your ideas