I have thought before about the "combined" priority, vs. the "per park" priority. I'm curious what others think.
Many people would tell you that you should just pick which parks you want to go to on which days, and then think about your fastpasses. But really, I think the choices are inter-related, so you are asking a good question.
Until Pandora, I think all of the Animal Kingdom FPs were the lowest priority. For our family, it was easy to have a great morning in AK without FPs. We could rope-drop Safaris, then head over to Everest and ride a couple of times with little wait. Then either KRR or down to Dino-Land. Then a mix of lunch, Boneyard, animals, one show, and back to the hotel for rest/pool/naps. A fun morning with no FPs and not much waiting. We could use our pre-booked FPs in Epcot or HS in the evening.
That plan might be outdated now. It might all still work, even with Pandora around, but obviously if you want to do the new rides, you probably want to FP one of them. Also, all the current rides, especially Safaris and Everest, may get more crowded as Pandora draws more guests to AK.
MK might also be low priority, since you can also have a great morning in MK with no FPs, and there are lots of fun rides that you can get as a 4th FP if you are hopping over. But the FPs for the 'mountains' and Peter Pan do save a ton of time, and they are hard to get as a 4th FP. One reason to put MK days later in a trip is to be able to book Mine Train FPs at the time you want.