According to Birnbaum's Walt Disney World 2007 book, Monday is the busiest day to visit MK. I would think the reason behind this would be that most families fly in on Sunday, so naturally the park that they want to see first on Monday is MK.
I
know I read somewhere that Epcot was least busiest on Sundays, but I just can't find the source for the life of me . . . I think it was in The Unofficial Guide to WDW, but I just can't find it in there. Anyway, Epcot has worked for us on Sundays and has proved to be crowd-free on Sundays 2 vacations in a row!
Wednesday is a great day to visit MK . . . most people who started their vacation on Sunday have already been there, so there's hardly any crowds. As for MGM and Animal Kingdom, I have no idea about crowd patterns. I'd just say any day that any of the four parks has extra magic hours is not the right day to visit that park!
Sure, ya get an extra hour in the park, but when it all adds up, you end up doing the same amount of attractions as you would for a day that didn't have EMH, because of the extra crowds. (For proof . . . remember on previous trips how those bus lines where is the mornings for parks that had EMH that day? And then the parks that didn't have EMH . . . well, the bus line is basically empty.)
So, stay away from parks that have EMH for the morning, Epcot on Sunday, any park but MK on Monday, MK on Wednesday (not Monday!)