Hyaluronic acid - BUT you have to use it correctly. It has to be applied to damp skin otherwise it doesn't really work. I spray my face with rosewater (it's an organic but inexpensive one from ebay) and then while the skin is damp put the hyaluronic acid on (or I also use Buffet serum which has hyaluronic acid in it as well as other stuff). Then I put moisturiser on top which locks it in, I use Weleda Skin Food which is awesome for dry skin although a lot of people find it too greasy, but if you use the hyaluronic acid first, you don't actually need that much moisturiser anyway.
Moisturisers don't actually do much to actively hydrate skin, they are more for preventing skin from drying out.