If buying an existing enameller does not qualify, then I think Piaget might be the only one. Consequently, the search for the "best enamel dial watchbrand" should be reduced to the one able to afford the best enamellers, or the one with the tightest quality control.
In my opinion, there are some techniques that have been further developed during the last century, and others seem to have lost in quality. An example of the first is, imho, cloisonné and champlevé enamels, which are now made in much finder detail, with thinner gold wires with more regular cross section, than they have been made in the 18th or 19th centuries. The champlevé cells are now more regular, those brought in shape by modern milling equipment permit stunningly complex outlines.
A typical example of the latter, however, is miniature painting. The majority of what is offered now, at breathtaking prices, is a pity, when compared with the amount of wonderful enamel miniatures produced before the 20th century, that are still available at reasonable prices at auctions or in specialized antiques shops. The reason, however, is easy to understand: Before the invention of moder photography, painting was the only way to preserve graphical souvenirs of sceneries, but, more important, of beloved ones. I am not speaking of the masters' paintings preserved in museums and art galleries, but of the typical everyday paintings (mostly portraits) that were present in every better household.
Mostly, these paintings were made in oil, water-colour, on paper or textile base. For travelling, this is not practicable, so enamel was the only way to make a painting resistant against humidity and temperature. Additionally, the enameling procedure permitted the application on all kinds of metal bases, even spherical ones, which were brought along during the travel anyway, such as pocket watches, mirrors, etc.
Adorning these with miniature pictures of the wife, husband, dog, horse, most favourite landscape or landmark, was a skill every average portrait artist worth his money had to master, and due to high demand they had the practice to stay perfect. Today, I have the impression, that only few currently active painters have this practice, since miniature enamel painting makes only a small part of their job. The few that are really good, have their order-books filled for the next years, and those offering quick delivery are not really worth the money that the final customers are charged for the finished product.
Just my two cents, of course.
Marcus