Well most of you might know it allready but so far best spot for fastest Nephelim valor boost to 5 i had found is fortified bunker/baracs place.
So the quest you wana run is siege brieker part 2(kill the siege brieker)
After you go in game teleport to Rakkis crossing and pick up tyrael.
Tyrael prevents all mobs from going into full health if you lure them close to respawn so even if you die a lot he will still clear it alone or with your help (unless they have vampiric).
If you are afraid you wouldn't be able to kill siegebrieke do not worry if you die tyrael will just continue killing him and finish him it might take longer but still better than nothing.
It can be found right after you leave city (north gate called Arreat gate).
You just go out of city follow the only road you can and when than you can chose to go to the battlefield do not go there. Right behind you can be found one of this two (fortified bunkerm/baracs).
I like fortified bunker more since there are no suicidal fallen maniacs. So i suggest going for bunker if you are not geared enaugh yet.
Clear whole lvl 1 and lvl 2 and you shud have around 3-5 NV stacks.
If you do not have all stacks yet you might try visit keep depts lvl 1 or 3 and look for some easy champ.
After you get 5 stacks. Open the resplendant chest in lvl 2 of bunker and head for siege brieker. with 5 NV you have garanteed drop of 2 yellow items so drops have some potensial.