You have to make a few test to find out which monster power is best for your characters. Sometimes it depends on what you are farming. If you are farming Keywardens, I guess we would farm at the most capacity of our character. But if we are aiming for Paragon Level 100, we would try the fast run possible. Items and Diablo 3 Gold farming would high MP but with easy run. Actually there is a system to test for free for you to find out which MP is best for your heroes.
Below is the blue post about this interesting test. You can go to DiaBro3 and fill in your battle tag. Then you would get Gear Score as well as the gear ratings.
You can find my test here. It says that my best MP farming should be Monster Power 6. As a matter of fact, I always farming alone and in a team with MP6. One of my Barbarian friend is suggested to farm at MP9. Usually he would solo at MP10. I would think that the test is interesting and useful. According to the gear score, I also find that my Inna's Radiance should my priority to be upgraded. My Inna's Favor and Inna's Temperance are of the top class. Now all I have to do is to focus on the items upgrade and farming.
This site says that most players are advised to farm below MP3. It seems that I am well above the average result. Now make your test and cast your vote. Which category do you belong to, MP3, MP5 or MP10? Share your results with your friends and invite them to take the test with you.