Nalgene 48oz Nalgene Cantene

I like these bottles because they're lightweight, easy to fill, and collapse as they empty. The 48oz size is great for use with the Steripen because you can use the standard 32oz dose of light - it's been proven effective in up to 48oz of water. That makes them extraordinary convenient and can help save on batteries. The wide-mouth is also standard Nalgene size so many other water treatment options would work too.

The one drawback is that they don't stand well on their own and are prone to falling over.