The Willow Patern has its origins in the Ming Dynasty under Emperor Yongle (thats said YONG LEE just as a matter of interest) in circa 1360. So to answer your question, Ming Dynasty China is the most valuable but even then the most expensive must be marked with Imperial marks for use in one of the Great Halls in the Forbidden City. Very few official records exist from back then as many of the records were burnt at the fall of the Ming Dynasty. The few records remaining detail some items that were made for the Emporer and as such these pieces demand extreme prices. Not forgetting there were tons of this china made for domestic use during the Ming reign and is still available today to buy relatively cheaply.
Next is old Delft ware as the Dutch were major importers of this china and to keep up with demand started producing the tin glazed earthen ware we know as Delf . Then Meisen discovered the secret of making the fine porcelain and started producing very nioce pieces. Some early Meisen pieces can demand very high prices as well.
