For me, I feel like there is no one "hottest" park, there are just spots in each park where I question my very will to go on.
AK- the entrance, especially the walk to the buses when you leave, gah!
Epcot- parts of the world showcase, particularly outpost area, I feel like the incessant drumming of children in the area is going to be the last sound I hear before grim death takes hold. Also that bridge between France and UK, you EARN your grey goose slushy through that.
HS- the walk to RNRC/tower of terror area. The concrete, the lack of shade, the concentration of people in one area, miserable. (The feeling you get when the AC hits you walking into RNRC, "OH MY GOD, WHO CARES ABOUT THE WAIT TIME. I'LL WAIT A MILLION MINUTES! Just don't make me go back outside!")
MK- it's a tie between new fantasyland, (it's like old fantasyland, but I swear it's 10 degrees hotter) and anything at all having to do with the Tomorrowland speedway. Be hot in line! Be hot as you board! Be hot as you ride! Die in the baking sun while you wait at the end of the ride to get out. And your prize for getting to the end? Stairs! (It's my nephew's favorite ride. I can't wait for it to be gone)