I almost always travel with a vapur that packs (mostly) flat, as noted above. I'll probably need to replace it soon - it's pretty creased in a couple spots and could crack soon. Would prefer that not happen during travel and in my carry-on....On some trips I use a stainless steel bottle, depends on what the trip is for. One main reason I travel with my own water bottle is that I'll use it during the trip, so for business trips I might want something a little nicer looking....

I use camelbaks (I have plastic and glass, different sizes and mouths) for daily use, at work and commute, but all my options are too wide to easily fit into my carry-on bags (and, for the glass, heavier than I want to carry)