Kobo rarely even gets mentioned in ereader discussions, but it is better than any other device on the market for myriad reasons, chief among which are that: (1) it reads just about every format ebooks have ever used; (2) It is arguably more easily independent of any vendor lock-in, walled-garden BS than any other ereader; and (3) it *functions* better than most ereaders in the first place.
Combined with Calibre, nothing can touch it. I have the Kobo Aura H2O with well over 1000 hours use and that device has definitely qualified among the 'cold dead fingers' realm of my possessions. Note that I avoid all non-locally-controllable anything like the plague, especially including walled-garden, proprietary-diseased clouds and their ilk, where I am supposed to trust some third party -- who holds my wellbeing in the lowest possible regard -- to act in my best interests. No thank you. The Kobo line is as close to Open Hardware ereaders as we are likely to see in the near future.