I have Source and Platypus water bags and have found them to be the best and toughest for me. No funk and low maintenance. And best of all no bad taste.
What is the preferred size, I know thats a loaded question but I always have bottles with other stuff like energy products so just going on day long hunts Im leaning towards the two liter, whats the general thought?
One tip on keeping a good tasting water bladder is between usages (at home) empty it out and store it in the freezer. Wont mildew and stays fresh tasting.
As for freezing in the later cold months use one with neoprene sleave on the tube. I find it helps in hot moths too. Keeps water from getting nasty warm.
I've been using the new Easton bladder that they make with Hydropak, the one with the tan hose (keeps things cool when warm and prevents freezing) and love it. Easy to fill, easy to remove the hose and clean. The tube with the covering is in addition to the bladder, but worth it when you're not sucking warm/hot water due to the sun - http://www.elk101.com/store/easton-hydrafusion-insulated-tube-kit