We just got back from our second trip to WDW and, for me, MGM could now be a one day park. I love RNR, TOT, and The Great Movie Ride, as well as Muppets, Beauty and the Beast, and Little Mermaid, but RNR and TOT are the only ones I care to repeat during the trip.
As for AK, it could be done in one day but I enjoy this park so much I have to go back and do it all again before the end of the trip.
MK, two days for sure.
EPCOT, at LEAST two days to explore each country. A half day for Future World.