I'm interested in using normal "Reynolds wrap" type aluminum foil as a radiant barrier. What are some measures of how much light and heat this would reflect under ideal and "normal" conditions? Are there significantly better types of foil out there? How much better-performing are the products designed for this purpose, such as foylon?
The reflectivity of household aluminum foil is quite good, but depending on your application there may be reasons to go with other materials. Thicker aluminum such as sheet or flashing can be polished allowing you to focus the light in a specific direction rather than just reflect it in any direction or diffusely.
Mylar (BoPET with vapor deposited aluminum) is a very good reflector and can provide better reflectivity than aluminum foil. Depending on the thickness it can be a bit more expensive per unit area, lower max service temperature, much more difficult to clean/polish for a long service life, and harder to conform to uneven surfaces.
It appears that "Foylon" is just aluminum foil or mylar laminated to some sort of fabric to give it better material strength. If your application provides some sort of structural backing, it would be no better than aluminum foil or mylar.