Platypus Big Zip LP
Platypus make some rather nifty hydration bladders for replacement or addition to your backpack, and the Big Zip LP 2-Litre has been on test here recently.
Firstly, I like the Platypus bite-valve, which is nice and chunky and easy to twist to an off-position so you don't end up with a dribble down your shoulder when your bladder gets hot or squashed. And it doesn't really taste of anything much, or taint the water, which is also a reassuring thing.
The whole thing is treated with Silver ions which make it antibacterial and keep you from getting a bad belly in the most unfortunate of places. I doubt these ions would protect against the mould which would accumulate in the bladder if you were daft enough to put juice in there, but they work well for water.
The hose itself disconnects with a clever little valve at the bottom of the bladder, which means you can take the bladder in and out of your pack without having to unthread the hose from whatever intricate pack design you have, and without fear of it leaking.
And when you do need to fill (or empty) the bladder, the opening is nice and wide to get under any awkward taps.
The closure on the bladder is a two-part affair. Principally it is a freezer-bag style press seal, which is good enough to hold water in the bladder. This is then teamed with a slider which holds it together and prevents bursting when you throw your pack on the ground at the end of a hard hike. It's very easy to use, and everything is tethered so it won't blow away.
And lastly, as hinted at by the LP in this bladder's name, the design is 'Low Profile'. None of the parts stick out very far, and the bladder itself has a welded anti-expansion bridge in it, so that when it's full, it stays fairly flat rather than becomes a tube. This should save space in your pack.