Everyone has different opinons. It's hard to say, but this is what I do:
For AK and MGM - we usually only visit these parks once during our trip. For AK, we either show up at opening and have a late (10:30) breakfast at Donald's or we show up after noon and stay until 5 when it closes. I have never found a bad day. We have been there on Friday and Saturday and never found it too crowded. (Off season only)
For MGM, we usually go after noon and hit a few attractions, have dinner and go see Fantasmic. Now, this is with a 4 yo, so we don't do Star Tours, RnRC or ToT, if you do, you need more time for those.
For MK and Epcot - we usually visit each at least twice. One day, we'll do morning, one day we'll do evening. I take advantage of EMH mornings at both these parks. If we hit MK for EMH morning, then we are out of there by noon or 1. Same for Epcot. Then, we do an evening or two to catch Spectro, Wishes and Illuminations. We try not to do these on EMH evenings. We never stay at either of these parks all day.
Hope that helps!