MK - Pretty much all the time, there's always something to do, certain areas look really nice at night, and during the day you will never be at a loss. This is the ultimate, anytime park. You'll never be bored.
AK - Note, this is usually skipped, but if I go it is more a morning park. It gets really hot as the day goes on and there's not really enough to do or a full day park (granted, I don't like roller coasters and the rapids is only good in the summer when it gets really hot).
Hollywood Studios - Afternoon is pretty good, can catch the shows, get something to eat. Night is nice as well, but unfortunately, they close some of the shows, so it is kind of empty in some spots (though, it's worth going there at night for Christmas time).
EPCOT - Afternoon is pretty good for the front portion of the park. Late afternoon going into evening is great for the countries. This way you can still catch the country shows and the great night lighting they have in each area that really adds to the atmosphere.
