The two most important facts to know are:
1) You have a better chance at finding high level drops in the later acts. So Act 4 > 3 > 2 > 1.
2) MF affects the # of affixes a drop will have. So if you have 300MF (Max) and get a iLV63 item that item has a better chance at being good than if you have 0 MF and get a iLV63 item.
What this means and what you need to do:
1) Put on the best possible MF gear set up you can get
2) Run the act so that you can breeze through
So while the best possibly scenario would be do obliterate act 4 with a max MF set, that is just not possible for most people. I currently faceroll act 2 with a max MF set. When I jump into act 3 I slow down substantially thus making it not worth it for me to make the jump. I could lower my MF but then my iLV 63's would have less of a chance of being good. And I am looking for perfect rolls. If I went down to A1 I would only clear it a little bit faster than I do A2 because I am owning A2 that hard. So in my specific case farming A2 is best for me.
Try A1 with a full MF set. If it's really easy try A2. If you are looking for efficiency do not farm any act that gives you difficulty.
MF gear swapping is also important.