For all the people saying AK is hot because of the plants, you should have been there when it was new. The plants hadn't grown out yet, and it was insanely hot, even for the animals. They'd hide back in the shade so it was hard to spot them in a lot of places.
I realize I am going against the crowd, but the only park my family has had heat problems is MK. At MK you are much more likely to find yourself in a crush of bodies, and all that body heat plus the weather has nearly caused fainting. We always plan cooling stops to get drinks and such, and MK is the only one where we have had to have emergency stops.