The most incredible moment (as well as sad one )for me: when you find Randall Clark corpse ,along with his mithical rifle and the last audiolog related to him, damn...he's got the most incredible story on fallout new vegas .

Followed By:

-ED's death in lonesome road
-Farewell of Dead Money
-Benny's Death (On Caesar's Camp,after talking to him )
-Harrold's Death (F.O.3)
-Dad's Death (F.O.3)

I would have to say your fathers death in fallout 3 was it for me. You spent all that time through hell to find your father and just when it looked like everything would be ok. Boom he sacrifices himself for you to carry on his legacy. Hits right in the feels everytime ...

That's a hard one. Well here's what I think:
-Gannon's Farewell
-Boones Quest
-Ulysses Farewell
-Muggy's fate (OWB)
-Liberty Primes Defeat
-Chris of the Bright Followers reaction at the end of "Come Fly with me"
-And ED's sacrifice.

not particularly sad to me but i found the part where you kill the overseer in fallout 1 was great at the time for those who started playing at fallout 3 here is a video of what im talking about

Id have to go with Lonesome Road EDE dying if you stop the nukes... best FNV character no doubt

One thing that gets me all teary eyed is at the end of Fallout 3 when it showes you the picture you took together with your dad on your Birthday. I don't know why but it just gets me in the feelings and all sad like you know?

Got to be the shotgun wedding in Fallout 2 and the invevitable death of your spouse later on. That and accidently (mostly) opening up on a baddy with Sulik the dumbass in the way of your assault weapon and getting his dang limbs blown off because I had too much to drink....

The most sad/greatest moment is easily in Fallout 3 when the main characters father kills himself to set back the Enclave and save the son. It was a very sad moment but it also inspired me to carry on his legacy and it gave the end of the game all the more meaning if the son or daughter gave up their life for the greater good also.

When James/Dad dies or when ulysses dies and you hear his reasoning

Saddest moment for me is when Harold is begging you to kill him

Incredible for me would have been blowing up Megaton. One of the most memorable moments.

Killing Amata's father without thinking it through on my first playthrough.

When I came back to the fallout 2 village no spoilers