The best factory depends on which replica watch make and model. One factory may have the best Explorer I, another may have the best Submariner. Anther factory may have the best Omega or Tag or AP.
Pick the watch you want then research the best factory for that make & model.
Check out the sticky posts in the Newbie FAQ Subforum please.
Its like cars: There is a best sport car brand, there is a best cabriolet brand, a best SUV brand....and so on