Fonts that work best for vinyl cutting will have smooth lines and enough width or thickness to it's shape to adhere to a smooth surface. For vinyl to be effective on the surface in which it is applied, the cut pieces need to have a substantial thickness in order to adhere.
Here are a few examples of fonts that will work well for cut vinyl.

 

The image below shows good examples of fonts that are not suited for cut vinyl. The lines may be too thin, too rough, there may be too many scattered pieces.