It all depends on what each person can tolerate. For us, dry heat is easier to deal with than humidity - it's like being locked in a sauna and having it cranked up.
I've driven all day (no A/C in the car) in WELL over 100 degree heatwave through death valley, CA and tolerated it better than high September humidity (80 to 90 degrees) in AK park. Actually, driving through death valley was SO dry we didn't even sweat. We had to keep stopping for water to keep hydrated and didn't need to use the bathroom either.
So, if you can't deal with the waves of heat from concrete, and direct sun beating down, one of the other parks may be worse for you. But if it's humidity without a breeze that wilts you, AK is definitely the winner in that category because of all the vegetation and the need to water it all.