Good nutrition is number one. A healthy horse has a natural glow that no product can put on. Flax seed and roasted soybeans are good coat supplements.
For some extra "oomph", I love Miracle Groom. It's especially handy in the winter because it cleans and deodorizes along with making them shiny. I like to brush them first, run a soft brush all over to smooth it out, then use a towel and a light misting of Miracle groom to polish the coat. I like my horses so sleek that they shine in the dark.
^^^ Clean, shiny ponies make me a happy girl.