The musky, funky odor of a dirty dog, which permeates everything they lie or sit upon. Making them go for a swim only makes it worse. The only thing that works is professional-grade pet shampoo and lots of warm water.
Their paws also smell like salty cornchips, because that is where their sweat glands are located. Pee-ew!
Don't go into my grandmother's Jeep Grand Cherokee... she has two Retrievers, and that vehicle is full o' fur and dogstank!