No question - July 1987. There were only two parks, it was 9,000 degrees at 10:00 PM, MK was open 'til 1:00 AM. The entire populations of Brazil and Japan decided to hit the MK the same time we did, rides were going down left and right and the monorail broke down at midnite. It was shoulder to shoulder sweat in umpteen languages.
The next day tropical storm Bob blew through and I had to fight off a South American travel group TWICE - they tried to steal our newly-purchased ponchos right out of my hand as I left the shop. When I grabbed them back, their tour LEADER tried to take them from me. Very strange.
I'll never go in July again.