Magic Kingdom: Pirates
I love the skeletons playing chess and the cannonballs and the long halls with all the turns. It really feels like you're walking through some ancient Spanish fort.
Runner-up: Space Mountain. I dig the long, dim corridor with the cool music and those weird hologram things.
Epcot: Test Track
It's cool and everything, but it's really the only line in Epcot worth spending some time in. Not sure why, but the lines in that park are really dull. (Think Maelstrom and Soarin'.)
MGM: Tower of Terror
The lobby is amazing, and I wish the line actually went through more of it. It's so detailed and evocative, but you spend so little time in that area.
Runner-up: Backlot Tour. I dislike the ride, but I love the gigantic prop room that you pass through on the way to board the trams. Again, it's a shame you pass through it so quickly that you have no time to really see much.
Animal Kingdom: Everest
Awesome line. I like how you wind through the village, then the store, then the museum.
Runner-up: Kali River Rapids. They did a beautiful job with that line. It's too bad all that detail doesn't extend to the ride itself, which is pretty disappointing.