I'm quite happy with my Asus X200M. It's compact, lightweight, and was pretty cheap, though I think it's discontinued now. It's small enough to stuff into a bag, in my bike basket, and cheap enough that I don't worry too much about damaging it.

I try to be platform-independent as much as possible. My "operating system" is Jupyter/Python. Knowing that I could switch to Ubuntu in a jiffy makes Windows seem much less annoying.