What's the best way to insulate a floor over a crawl space?
The 'best' option is full encapsulation and insulation. This is the recommended method today. The primary arguments are:


humidity levels in an encapsulated space are much lower than vented spaces
due to it being encapsulated, it's much cleaner which can improve indoor air temperature (no pest droppings, dust, mold, pollen, etc. migrating up through the floor)


However, if you don't want to go that route, there are recommendations for insulating just the underside using rigid or spray foam. Building Science Corporation has recommendations.