Choosing the best mustache comb requires some basic knowledge about the types of combs that are available and a little research. It's important to know in advance that the best comb for most types of mustaches usually will be small with narrowly spaced teeth. Certain facial hair is too thick or curly for the finest combs, but mustaches, in general, require a fine comb for proper training and grooming. These combs might or might not have a handle.
When growing a mustache, it's helpful to comb it often to train the hairs, especially if you plan to style your mustache eventually. To effectively train the hair, you need a comb that has fine enough teeth to really grab onto the hairs and pull them. This can be painful initially, so it might help to have a wider-toothed comb for the first few passes each time you groom.
A mustache comb usually is much smaller than an all-purpose hair comb or even a beard comb. This smaller shape allows for closer, higher-precision combing and can help with styling. Before styling a mustache into handlebars, for example, it's very important to develop a strong part with the mustache hairs trained to fall on either side of the mouth. Only a very small and fine comb is ideal for this, but you also want to make sure that the comb is strong enough to pull the hairs, so it shouldn't be too flexible.
A very small mustache comb is also easy to carry around, which is important when you need to groom your mustache often. Most mustache combs have a handle, but they usually are still small enough to remain portable. If this is a notable concern when choosing the right comb, you might want to look for a folding comb. Some mustache combs can be folded like a switchblade knife.
Whether you go for just a basic groomed look or style your mustache into a chevron, pencil, walrus or handlebar shape, choosing the right comb takes a little research. Compare shapes and sizes and realize that the pictures that you see online might not be true to scale. Read the actual dimensions if you're looking for something small, and try to read reviews when you can find them.
Lastly, know that some mustache combs are also designed for beards. As such, they might not be fine enough even though they are advertised for mustaches. Try to find reviews written by people who actually have mustaches so that you can confirm that a given comb is truly adequate for the job. When in doubt, it might be best to stick with a comb that is advertised as only a mustache comb.