I cannot find a reflective white, or near white paint with a translucent finish, and it has to be exterior paint for painting street signs.
First of all, reflective road signs are created in a multiple step application. First, the color to be used is laid down. Once this layer dries, then a leveling coating is applied followed by sprinkling small glass beads on top of it. The glass beads need to be uniform and as even as possible for the best reflection.

Secondly, The most translucent white pigment is the Zinc White, available in Heavy Body (tube acrylic thickness) and the Fluid Acrylics. Same pigments and pigment-loads, just different viscosities. I personally find the Fluids are excellent for signage and lettering, as they do not require any dilution to flow from a lettering brush.

Lastly, the GAC line of Specialty Polymer Mediums offers various acrylics to extend and modify paints for specific reasons.

Which white will best "glow" under a black light. I am using Titanium White on black leather canvas. It barely glows. Can I do anything to have it stand out more?
Titanium White pigment will not glow under black light as you mentioned. You need to have a white paint with some sort of Phosphors in it to make it glow white. This is what makes white garments glow when you go onto a room with black light, as the detergent manufacturers use the phosphors as a "color brightener". IF you hunt, this is why you don't want to use commercial detergents on your clothes, because the deer see it like people see Fluorescents. We have a custom product called Fluorescent Clear. It emits a bluish white color. The standard product, GOLDEN Phosphorescent Medium, emits a greenish glow under black light. You might want to try Sign Companies for a Fluorescent White Paint, however, if you are painting on a jacket, they may crack when flexed.