What portable drinking container do you use?

Concerns over soft plastic bottle leeching chemicals are misplaced. Most soft plastic bottles are made of HDPE and LDPE, both of which are relatively safe, chemically, and should be reusable for quite a while. The recent concern is over PVC and polycarbonate bottles (i.e. Nalgene bottles). These are the plastics known to leech Bisphenol A. (I was also told by a friend that a good way to avoid BPA leeching into your water is to avoid getting it hot; the validity of this claim I can't be arsed to assess.)

That said, there is still the valid concern about reusing a standard water bottle too much because of germs. Due to the finer details used in the designs of commercial water bottles, they have a tendency to hold on to food particles easier, which fosters bacterial growth. Some paranoid people will say that you should hand wash your bottle every day, but I find a good washing once a week should suffice. Also, the washing should be done by hand, because a dishwasher is unlikely to be effective on something with such a small hole.
One of the reasons (I'm guessing the primary reason too) that they make reusable water bottles with such large openings is because washing them is much easier.

Jam it back in, in the dark.