Crypt of the Ancients Farming Location
Crypt of the Ancients is a small dungeon located right off of The Festering Woods in Act 1. Most players clear out this cave when they are collecting Nephalem Valor and don't ever give it a second look, however on a really high Monster Power this cave is actually gear to farm. I really only recommend farming here on Monster Power 10, but 9 may be fine too. If you can't handle this high of a MP you may want to just find another place better for you to farm.
In order to farm here you will need to select The Broken Blade quest in Act 1. The first part will have you enter The Festering Woods and after that you'll have to find two places. You will want to find Crypt of the Ancients and enter the dungeon which will give you the Checkpoint you need to farm this place. After you clear this dungeon out and leave the game every time you enter it again you will simply be able to hop into your portal and come right back to CoA.
The thing that is so good to farm at this location is the infinite spawn of Skeletons. There are Skeletal Summoners here who summon tons of squishy skeletons. On Mp10 each of these skeletons have an insanely high (it may even be 100%) to drop something, it could be Gold, A Gem, A blue, a Health Globe or even a Legendary. That's what makes this location so amazing to farm. With all of the mobs you kill here having a chance to drop loot, you're going to be getting quite a bit of it!
Not only will you get quality and quantity of loot from this place you're also going to be getting some decent experience a run here too. Each run will net you about 3 mil XP which isn't that much but considering each run will take around 3 - 5 minutes to complete it adds up quick. Keep in mind you don't need to kill the Champion mob inside of here, you can simply just kill all of the weaker skeletons and then port out or leave the game. In fact most of the time I ignore the champ mob because he takes entirely way too long to kill.
Also I usually don't clear out all of the mobs inside of here before I end up leaving. I kill about 90% of what is in here until it becomes too tedious and then I bail. How you want to do stuff in here is entirely up to you. If you'd like to watch a few runs to see exactly how I do things below is a video I made featuring my CoA farming guide.
That's about all there is to farming here. Below are pictures of my before and after XP from a single run here. Note that I do not use a Leoric's Signet nor do I have a Hellfire Ring that I was wearing at the time.
XP at the start of one run
XP at the end of one run