Amazon doesn't require registration or even wi-fi setup. You can use it purely sideloading with Calibre over USB without ever connecting it to the internet. Caveat, no collections support unless you jailbreak which may not be available on one's device or firmware.
Kobo requires signing in with an account. Same goes for Nook, I believe.
I think there's a way to skip the Kobo sign-in. I couldn't access USB storage before initial setup and login though, so short of physically dismantling the Kobo to take out the microSD, I don't know how one would be able to do it.
My primary readers are jailbroken PW3 and PW4. I like the rounded corners and the fact that they're relatively cheap to acquire in the US due to frequent sales. I use both the native reader (better text search) and
KOReader (epub and cbz support) on my Kindles.