I love Cowboy Magic, Mane & Tail, and Eqyss.
Mane and Tail makes their coats super soft, but I usually use Eqyss or Cowboy magic to really scrub the dirt off. For white spots/legs, I love the Orvus shampoo. It really whitens and is very bubbly.
A good shampoo for making your horse shiny is the Man O' War shampoo which is less popular that those other ones but is good despite the cheap price.
For a horse with dandruff or rainrot, I like the Cowboy Magic Krudbuster, but especially love the Extreme brand of medicated shampoo. It's about 20 dollars but can be diluted. I used this on my horses face, mane, and tail, and it removes all of the dandruff with a little scrubbing. Good Bathing!