Elector PSP. It's the best. But it's expensive. It's highly concentrated, so it will save you money in the long run.
I like it because it mixes into a liquid you can spray it on your chickens or you can dip them. You can use the same spray on the coop and get into all the cracks and crevices. The best thing is it's a biological pest control and is non-toxic and there is no egg withdrawal. Spinosad is the active ingredient, and it's used by many of the largest poultry egg laying operations.
Permethrin dusting powder is the next best thing, and it's much cheaper, but messy, and it's hard on your respiratory system as well as the chickens. It works, though. You may need to repeat the treatment in a week or two. The Elector PSP lasts much longer.
By the way, lice are probably what you are seeing on your chickens. Lice live on the dander and dead skin, but are harmless to the chickens. Mites are another story. While they don't live on the chickens, they do come out of the crevices at night and bite the chickens, making their lives utterly miserable.