Animal Kingdom 100%.
On our first ever July trip, this was the only park that caused me to have a heat "melt down" in the afternoon. We just couldn't find enough indoor things to do during the hottest part of the day.
Some posters above mentioned Epcot and HS but we found so many indoor things to do at both of those places that we were able to alternate heat with AC. At AK there are huge parts of the park with no AC venues. Plus if you happened to arrive 5 minutes late for an indoor show at AK you are out of luck for at least an hour. They also stopped a lot of the shows in the late afternoon. We did not find this to be the case at any other park.
We really like AK, in fact it is my son's favorite park but it was NOT my favorite place to be at 4 in the afternoon in July. We usually arrive right after opening at parks and stay till close. Yet, if we ever visit AK at that time of the year again, I am taking the recommended "break" from that park in the afternoon.