Kobo, with either stock OS or KOReader (I use this, in part because the font size can be easily increased for my daughter who so far needs text larger than stock) or Plato.
Weirdly, about half of them have developed a problem after about 5-7 years, whereby they intermittently stop charging. Replacing the battery doesn't seem to fix it. Might be a problem with the soldering of the USB connector to the PCB?
As a bonus they are Linux based, and you can do fun things like replace internal SD cards with bigger ones, login using telnet and install new applications.