SAFD:
Splash Mountain-Always. It was “the ride” on my first visit in the early 90’s. My siblings and I were aware of nothing at WDW upon our first visit other than Splash Mountain. Our first day at MK there was an afternoon rain shower and we rode probably 5-10 times in a row because no one was getting in line. It’s the only vivid memory I have of that first trip.
This has a couple different dimensions for me. The first I view as a question, “if every attraction in the park had a one hour wait, what queue would I pick?” (Since this is purely hypothetical, it’s going to be exactly one hour whichever you pick) Behind Splash, I’d go:
Flight of Passage
Toy Story Mania
Kilimanjaro Safari
Rock ‘n’ Roller Coaster
Expedition Everest
Soarin’
The second considers the average wait times I’ve experienced in my trips to the park and my enjoyment vs. queue time. (I hate waiting in lines; don’t we all) Using this metric, the following are some of my favorites:
Gran Fiesta Tour (usually walk-on)
Under The Sea-Journey of the Little Mermaid (usually under 10 minutes)
Haunted Mansion (15-20 minutes, my experiences may differ from others here)
Star Tours (under 15 minutes)
ETA: I don’t know how to factor nighttime shows, but Happily Ever After with a good viewing position is my favorite thing at the parks and I enjoy the Festival of Fantasy parade quite a bit as well.