Fushigi Yuugi Kanzenban (Perfect Version)A recompilation of the original manga into 9 vols. Each vol. contains twice as many pages as the original version, are A5 size (6" x 8"), and are printed on better quality paper. All pages that were originally illustrated in color are in color here with the exception of the "Harem of Men" manga. (But since that was printed in color in the 1st artbook it's not a big deal that it isn't in color here.) The series also contains new extras as well as newly done character profiles. And if you look under the removable cover you'll find some character sketches underneath.
On the con side, the Kanzenban version doesn't include the Watase Yuu freetalks or some of the original extras (Most notably the Fushigi Akugi) and it does cost more than the original version.
If you're only going to get one version, this is the one to get as its pros far outweigh its cons. However, die-hard FY fans who can read Japanese will probably want to own both versions.
(Side by side comparison)
Watase Yuu | Fushigi Yuugi