Which area in Act 1 can I gain Nephalem Valor the fastest?

Solution 1:

Nephalem Valor no longer exists in the game. You cannot farm it at all, no matter where you go.

Solution 2:

I would say the Cemetery of the Forsaken.

With a chance for a unique to spawn, plus three very small dungeons, two of which are one level only with exit teleporters, you should be able to find 5 stacks in a very short amount of time.

Solution 3:

I recommend the Cemetery of the Forsaken and the Warrior's Rest and Crypt of the Ancients (the two caves/crypts you need to enter before you can enter the drowned temple).

The latter two only if you haven't got 5 stacks from the cemetery alone yet.

Solution 4:

Another very good area would be Festering woods. The area is quite small and usualy has 3 elite groups by itself. There also are 2 very short dungeons with 1 elite group in each.

A lot faster than cemetery of the forsaken since the area is much, much smaller. However, the trash deals more dmg, so you have to have a bit better gear.