There is no single weed killer that is best for all lawns due to the varying types of weeds and grass that can be present. There are three types of lawn weeds, each require a different type of herbicide. There are also three distinct types of herbicide.
Preemergence herbicides kill seeds and seedlings, and is the most common type to use against crabgrass. Postemergence herbicides are used to combat weeds that are already growing, they kill via contact or by circulating throughout the plant. Weed-and-feed products can be used to both fertilize the lawn and kill weeds at once, although they must be used at the proper time of year for the maximum effectiveness.