Start with capacity and carrying style. A larger bottle means fewer refills but can be heavy when full, while a compact one may fit more easily in a bag. Measure your cup holder, backpack pocket, or bicycle cage if the bottle needs to fit a particular space. Think about where you will refill it and whether you prefer sipping through a straw, drinking from a wide opening, or using a simple screw cap.
Materials have different strengths and care requirements. Stainless steel can be durable and may help retain temperature when insulated, but it can add weight. Plastic designs are often lighter, though you should check the product's care directions and intended uses. Glass offers a different drinking feel but requires more protection from impact. Inspect lids and seals closely; a comfortable bottle is less useful if its cap is difficult to clean or prone to leaking in a bag.
Consider how easy the bottle is to wash, including any straws, valves, or narrow necks. Confirm whether it is dishwasher safe rather than assuming every component can go in the machine. If temperature retention matters, compare the maker's guidance and remember that performance depends on use. Choose a design you will carry and clean consistently. The most sustainable bottle for your habits is one that gets regular use over time.