Bubba is right on the mark. Do not go to MK on a Monday or Tuesday. Everyone starts their vacation there. Try and hold off until Wednesday and you will be so much better off.
We normally do MGM on Monday, EPCOT on Tuesday and then MK on Wednesday and then the rest of our time is pretty much spent at EPCOT.
Although this trip we are going to go to AK. Seems like we never have time left for AK.
It depends who you are going with. Do you have children in your group? If so, then head to Fantasyland first.
For us, we hop on the train and head back to Splash Mountain. Ride it about 4-5 times before the line starts to get long and then move on.