Source has really upstaged the competition with their WXP Hydration Bladders. At first glance, they appear to be no different than any other hydration bladder on the market but take a closer look. Not only can these be filled via the typical screw off wide mouth opening, but they also have a clip at the top that can be removed. Once the clip is removed the entire top of the pouch can be opened up wide enough to stick your hand inside the bladder! That means easy filling and, perhaps more importantly, easy cleaning.
They are priced very competitively and come in a wide variety of sizes. There are civilian colors and more subdued military colors available.