Looking for a bag that will work with my Deuter ACT-Lite 40 + 10 backpack and I can use it to fetch water from the stream, bring back to the campsite and use for cooking. Do any of these fit that bill???

My personal preference is the 6L. This size does well for the purposes you stated and packs small when not in use. It also uses a 64mm threaded opening that fits most filters, especially the MSR Miniworks EX. This combination is hard to beat. I would also get the MSR Spigot Cap that has an on-off valve, making it super user friendly at camp; just like your kitchen sink at home!The only question I have is whether you want to carry water in your pack, especially in a 40L pack. If this is the case, you might want to get the 4L size.