Glass prevents anything unwanted from getting into your fluids, and most plastic bottles are classified as BPA-free. However, rememeber to wash your bottles regularly to keep them safe and clean.
Metal bottles made from stainless steel or aluminum are also safe, but each material comes with its own pros and cons, from how durable they are to how easy your bottle is to wash or how heavy it might be.
Glass bottles might look great, but they're easier to break than durable stainless steel models, which are also great at insulating your drinks. Plastic is arguably most lightweight but can leave a taste in your mouth and don't fare as well in the wash.