The best, easiest, and least toxic degreaser is hot water and dawn dish soap. I have tried simple green, gasoline, engine degreaser, bike specific products and the hot water and dish soap works the best. I believe it must be because of the surfactants and detergent.

Most of the other chain cleaners are just degreasers. But you aren't just trying to remove grease/oil but road grime which may or may not be soluble in oil and therefore may be soluble in water. You need to not only remove the grease which the detergent does well, but you need to remove other particles which may be soluble in water. The surfactant allows the detergent to get into the nooks and crannies.