Lil' late answer but really there isn't a single end all be all font. Though pretty often for original designs I tend to favor Arial Black for big and generic texts, then Commercial Script or times new roman in bold for stuff like mottos. A good way to make it interesting is adding drop shadows and outlines, and a tilt of a few degrees, examples below!
Example of drop shadow + tilt
Example of Tilt, then outline, then dropshadow
Examples of typical texts on police cars in various fonts
For US fire/medical, i would use times new roman style fonts in bold, and for euro style I'd use more Arial type ones