That's a very tough decision to make, but I think I'll have to pick the Haunted Mansion. All of the pre-shows mentioned are really good, but I think the stretching room has stood the test of time. It's classic.
The Tiki Room one for us and some friends is a big inside joke...I will try to explain...
When we went the first year it was open we went with some friends. Well one person we were with worked for the Talent Agency William Morris. Well in the preshow notice that the birds names are William and Morris (this is on purpose. Disney has strong ties with the agency). Well our friend was on his way out of the company, he had found a new job. All he wanted to do was not think about The Agency (they were giving him a ton of work to finish before he left). So he was very happy to be in WDW to relax and not think about the work. Well when we got to the MK the Tiki Room was one of the first things we did. It was so funny when we were watching the preshow and the birds names were William and Morris.
You probably don't think it is funny, but like i said it was an inside joke.
I think the preshow at Cranium Command is one of the best. It is so Funny. "Where do you think you are DISNEY WORLD!"
There was a tie for me! The Muppet-Vision pre-show is funny but I basically know all the lines of it and I worry the show will get old for me. The DINOSAUR pre-show is hilarious.
I had to vote for Dinosaur mainly because it's one of my favorite rides at WDW. I think the que room and the video do a great job of setting the appropriate "museum-like" tone. And the shift from dry museum vibe to functional high-tech when you go into the loading area is great. I alos thought it was very clever to include the cast members overseeing the in-ride portion in the loading area (the control room to the left of the launch area).
Maybe it's just me, but I consider room number one of Alien Encounter to be the preshow. You know, the one with the TV screens over head. Once we move into the room with SIR and Skippy, for me, that's part of the actual show. It's just part one of it. I mean, can you really have a pre-pre-show?