Are you talking internal products or external, like undercoating?
Since I decided not to put back the undercoating on my car, I used a couple of things. First, I used dynamat on the interior floor and interior rear wheel wells, the car originally had a tar paper on the floor, and the repops are not very good. Second, I used stock shoddy pad and insulation on the firewall and kick panels plus was able to find the stock tar paper product for the sail panel area.
Third, and maybe to help answer your question, I used a product from Evercoat, rubberized undercoating/sound deadener to spray on the inside of the outer door skins and interior parts of the rear quarter panel skins, and seems to work very well.
Hope that helps, and also some others have used various products here so I am sure you will get more feedback.