Plitvice Lakes, Croatia, followed by pretty much ANYWHERE on the Dalmatian coast.
Iceland, anywhere (out of this world "did we just land on Mars" country).
Fraser Island, Australia.
Swiss Alps in Winter.
Swiss Alps in Summer.
Bah, you said just 1. Impossible to choose and every time I travel, a new place pops on the list of 'most beautiful'. Do yourself a favour,
EarthPorn: Mother Nature in all of her succulent beauty. for ideas!