[Sirtel]

If you want to sideload and manage the books on your device solely via Calibre, there's nothing better than a Kobo. Kindles are not even in the same league in this regard, their library management is very limited in comparison (I have owned and used Kindles before I switched to Kobo).

If you want to keep your books on a remote server and sync the reading progress across several different devices, then Kindles and Pocketbooks may suit you better than a Kobo - Kobos don't sync sideloaded books over different devices, while both Kindle and Pocketbook offer such a service.

Everything else is a matter of taste.