Assigning a single book the title of being the most beautiful in the world is, of course, apocryphal, but my vote would nevertheless go to an exquisite tome known as the Kelmscott Chaucer.
Formally known as The Works of Geoffrey Chaucer, it was designed by William Morris, the brilliant late 19th century English artist, writer and socialist, and its 87 woodcut illustrations were done by friend and contemporary Edward Burne-Jones. Both were closely associated with the Pre-Raphaelite movement, which in turn begat the Arts and Crafts movement.
To a modern reader accustomed to the functional sans-serif type faces of the Internet Age, the Kelmscott Chaucer has an astoundingly decorative richness set off by foliated borders and a gorgeous ornamental binding that varies according which printer produced it. The clarity of the printing is astounding and typical of Morris's Kelmscott Press, which he said he founded "with the hope of producing some [books] which would have a definite claim to beauty, while at the same time they should be easy to read and should not dazzle the eye, or trouble the intellect of the reader by eccentricity of form in the letters."
Some 425 copies of the Kelmscott Chcaucer were completed by 11 master printers, and thanks to Morris's personal magnetism, the entire edition was sold out before the books were finished and issued in 1896. Copies can be had for as little as $40,000 and for as much as $150,000 depending up the condition and binding. Inexpensive facsimiles also are available.
Buying a copy is, of course, well beyond my modest means, but I have access to a Kelmscott Chaucer that falls somewhere between the two extremes in value and I never tire of leafing through it.
As a bibliophile friend remarks, the Kelmscott Chaucer is not a rare book, just a pricey -- and beautiful -- one.