It really depends on what you're looking for...MGM has a nice display of "better" Disney watches, I don't know the name of the shop, but as you enter the park, it is the shop that is in the second block, on the left hand side...it carries the menswear and ladies wear (Disney Clothiers, maybe?) Custom painted watch faces are available at MGM as well, at a shop on the right as you approach the Beauty & The Beast show area.
DtD has a good selection in the World of Disney, and yes, there is a shop on Main Street at MK has also has a good selection.
If you're looking to go with a less expensive watch, many gift stands/shops throughout the parks will carry them. If this is to be a "special" watch for you, I suggest you take your time and browse around, while a lot of stores carry similar merchandise, they do not always stock identical items, and you may find that "perfect" watch as close as your resort gift shop.