Depends on the use case. I default to my 1L Nalgene or my metal 0.75L generic metal bottle I got at a museum for daily use whilst sitting at my desk at work. For hiking, I prefer the larger capacity Nalgene (I think 1.75L), and possibly an additional 1L for longer hikes. For cycling, I have a couple squeezy plastic bottles that fit nicely in my bottle cage.
I do like the idea of a glass bottle with the rubber/silicone guard thing and thought about getting one, but decided to stick with Nalgene when I had the replace my old 1L.
They don't, you're just obligated to report your global income as a US citizen. This isn't a big deal if you plan to never go back to the US to live there, but if you do (or aren't sure), this becomes a major issue because all those years you didn't file your taxes with the IRS makes you effectively a tax cheat whether or not you owe any taxes at all.
If you pay income tax to another foreign government (that has a tax treaty with the USA) and you earn below the threshold for an individual or a household, then you don't pay taxes at all. In order for the IRS to determine this they need to know how much you made. I don't know what the actual threshold is for individuals, but in the 5 years that I've been living outside of the US, I haven't paid a single cent in US income tax. It's a pain in the ass, but whatever, you get used to it.