I've got a Camelbak "unbottle." I think that's the right name; it's the one that comes insulated. I like it because it really does keep the water cooler on a hot day. The insulation also helps prevent punctures, which is important if some of your packs are older and don't have a hydration pocket. It does of course fit in a hydration pocket as well.

I like its versatility:
-I can use it in the "usual way" in a hydration pocket.
-Because it's protected, I can also put it in packs w/o a hydration pocket.
-It has clips, so I can run run straps through the clips and secure it on the back of my pack if I need more room inside the pack.
-Also with the clips, I can put climbing webbing or similar straps through the clips and wear it directly on my pack. If I'm doing some kind of a "fast and light" outing in dependable, good weather, I can wear just a fanny pack and still have a good supply of water (comes in 2L or 3L size) when I add my Camelbak. A lot of the time (pre-Camelbak) when I was out with just a fanny pack, even a good sized one, I had trouble carrying sufficient water for the hot dry weather we often have here in California.