That's a real tough question! It would be hard to pick just one disc. I was thinking the other day about which "land" is my favorite, and even that is kind of tough. A very close race between Fantasyland and Critter Country!
Ultimately, I'd go with Fantasyland by a nose (I LOVE every track from Fantasyland quite a bit, esp Small World, Darkride Suite, and King Arthur Carousel!) , followed CLOSELY by Critter Country (2 CBJ shows + Splash Mountain and Pooh give Fantasyland a good run for it's money!)
In third I'd go with Adventureland because it has the Tiki Room soundtrack and the CD builds up to this PERFECTLY by including Barker Bird and the Garden Show! And I LOVE Swisskapolka!
The rest after that are really close as there is NOTHING on this set that I dislike! I'm generally not into Tomorrowland quite as much as the more fantasy and nostalgia based content from the other lands, but having said that, the fantasy and nostalgia-based America Sings, from Tomorrowland, is one of my favorite tracks on the disc!
But in general I find myself listening to disc 1, 2, and 3, and of course the beginning of 4 (IASW and Toontown) more than the last 2.5 discs. I'll probably play disc 6 A LOT during the Christmas season!
If I could only take one disc to a desert island, I'd probably just burn my own "highlights" disc, which would include ALL of Fantasyland and the AA musical shows from the other lands!
David