In honour of this very board -- I'd have to say it's the only tour we ever took at WDW -- the DVC tour!! It must be the only tour that ends with free ice cream and the potential for a lifetime of great holidays!
I have taken The Keys to the Kingdom, The Magic Behind the Steam Trains, and the Seas Epcot Tour. They were all special in the sense that I learned something, and we went behind the scenes and all that, but I would do the Seas Epcot Tour again in a minute. That involves swimming in the tank at the living seas with a breathing apparatus, so it's like snorkeling, but there's an air tank. You are not allowed to dive, it's not scuba. The money that you pay goes to Disney's Conservation fund.
We loved the tour that takes you behind the scenes in Epcot, MK, and MGM. I think it's Behind the Magic or something like that. Spent almost a full day and didn't have to waste a park ticket. Lunch at Mama Melrose's is included and it was awesome!!
So far it's been the Backstage Safari tour at AK, but AK is my favorite park, so that's not surprising. But I'm hoping to get to compare it to the Segway tour soon.