Vapur Bottles, because normally I use stainless or glass. But I have been using them like crazy this Summer and here's why: you can freeze them. Being able to freeze them means you keep them ready to go in the freezer, grab on on your way out, and hot Summer weather does not interfere with having a cold beverage in hand.
Not only that, but they are fabulous for packing in a lunch to keep the surrounding food cold and for packing in the cooler for picnics and camping. when it thaws, you still have a cold beverage to drink unlike those frozen blue things you buy to keep your cooler cold. I've even used them to soothe headaches (they fit just right at the base of your neck).
They are great for travel, as you just roll up the bottle and tuck it away to reuse later when you're finished. They come with a removable carabiner to attach to your backpack or to hold the bottle compact after it's rolled up. They take up very little space and their weight is negligible. They're BPA free and perfect for those times when a bulkier bottle won't work. They come in several colors and are available HERE.