My kids on our last trip were 8, 5, and 3. So, of course at least 2 days at MK is a must.
We also always do at least 2 days at EPCOT. We all love it there. It usually takes almost a whole day for us to cover Future World. Then, on the second day we do the highlights from Future World (Soarin, Test Track, Figment) and then hop on over to World Showcase. It still always seems like we don't have enough time at the countries. And now this year they have the KimPossible missions which look like a lot of fun.
AK is a one day park for us, but you must plan the showtimes out well if you want to fit it all in.
MGM is rising up on the list for our family now that the kids are getting a little older. It can also get a little hard to fit all of the shows into one day unless you really plan it out well. We actually spent 2 days here last trip because the kids wanted to do some stuff over again, but we did manage to see most of it in one day with some careful planning.