Remarkable 2 is the only one I have experience with, and I recommend it highly if you are comfortable exploring the community APIs and tools. It uses a proprietary cloud platform by default, which may not appeal to folks, but you can use ssh over usb to avoid their cloud entirely. I use a script based on
https://github.com/subutux/rmapy to send the tablet pdf files with one command. The reading experience is excellent and the writing surface/pen are great for marking up documents as well. It's a magnificent paper notebook replacement that you can fairly easily put pdfs on. If you need on-tablet web browsing, other apps, or state of the art epub reading, I'd look elsewhere. For getting reading material off a backlit screen as though it were an endless stack of paper with arbitrary content, it's phenomenal.