Among the worst was a downpour when we were at one of the BC area hotels and had a World Showcase ADR. We had no choice but to walk all the way in heavy rain.
A second bad one was a time a member of my family slipped on wet pavement trying to make a bus on the first day of our trip. It was barely raining, but the fall on the sidewalk caused a large cut and a sore ankle that lasted several days.
Best is probably while sleeping at the hotel.
A funny one was back when they projected photos on the castle. WDW said they were daily photos, but it rained that WHOLE day. In all the real photos, people had the wet rat look. About half the photos though were CLEARLY taken on a sunny day, featuring highly photogenic families. It was funny.
(Early 7DMT Photopass videos had the same kinda thing if you rode it after dark: you'd get a video that was half you/night footage and half daytime footage.)