Which undercoating is the best?

Which undercoating is best depends on whether you mean house painting or car rust protection and your goals. For house painting, best undercoating is primer suited to surface, oil based or shellac based primer like Zinsser B-I-N or Cover Stain for bare wood with knots to block tannins, PVA drywall primer for bare drywall, bonding primer for glossy surfaces, and exterior wood primer for exterior wood, as these seal and provide adhesion. Combined primer undercoat from reputable brands like Zinsser, Benjamin Moore, or Sherwin Williams is best general purpose. For car underbody rust protection, best type depends on vehicle condition, rubberized undercoating like 3M Professional Grade or Rust-Oleum Rubberized is best for new cars with clean underbody providing hard physical barrier against stone chips lasting 2 to 5 years. Polyurethane or epoxy based like POR-15 is best for restoration and long term 5 to 10 years durability. Oil based or wax based like Fluid Film, Krown, NH Oil Undercoating, or Woolwax is best for old cars and rust belt because it creeps into seams, displaces moisture, does not trap rust, but needs annual reapplication. So best overall for new car long term is polyurethane, best for salty winter and existing vehicles is Fluid Film or Krown oil spray, best for house is surface specific primer.