Just one? Probably Rivendell.
Runners up:
Frank Herbert's Arrakis, to learn to ride sand worms.
Iest, to hang out with Lord Julius and drink with Cerebus the Aardvark.
Dave Hargrave's Arduin, to see a stampede of Thunder Bunnies.
Barsoom, mostly to meet Dejah Thoris.
Poseidon, the...