There's no perfect answer to this onethough for my money, it's the Guyot Designs stainless steel Backpacker bottle. That's right, a stainless steel bottle over an aluminum or plastic one. Here's why.
Plastic: The recent BPA scare was much ado about very little. Probably no one was really being harmed by the miniscule amount of exposure they were getting. But still, polycarbonate plastic (#7) is almost never recycled, it does degrade with use and age, and its manufacture creates a sasquatch-sized carbon footprint.
Aluminum: Yes, my kids carry aluminum water bottles in their lunchboxes every day. But aluminum takes a scary amount of energy to extract (from bauxite taken from the ground) and produce. On the plus side, it's nearly 100 percent recyclable. And many aluminum bottles made by outdoor companies are lined to prevent any kind of leachingso you can rest easy about that tenuous link with Alzheimer's.
Stainless steel: Sure it requires a lot of energy to manufacture, but nearly two-thirds of it comes from recycled content, according to the International Stainless Steel Forum. The downside is that more than one-tenth of it is chromium, which can leach if exposed to acidic materials. But we're talking water here, so it's not really an issue. All things considered, I give the advantage to stainless steel.
Why the Guyot Designs Backpacker. It's high-grade stainless steel, its 63-mm wide mouth is compatible with water purifiers, and the company buys carbon offsets for every one of these it makes, so the bottle is supposedly carbon negative. (Although, if you've read my past columns, you know what a bunch of bunk I think carbon offsets can often be.) All for the low, low price of $24.95.