I got my partner a Kobo Clara Colour. In my view, it has a few killer features: it's not tied to Amazon in any way, it works well with any public libraries that offer e-book lending through Overdrive. It lets you get books from your library without using a computer. It also has a color e-ink screen and seems to have adjustable backlight color.

I have a boox palma 2. For me the killer feature is the size. It being phone sized, I take it more places and read more often with it than other ebook readers I've had.

I've also had a few Kobo devices, and loved them all, but for me a phone sized ereader has turned out to be the sweet spot. Since you mentioned it, it does have warm/cool backlight controls.

Kobo and Boox both place no restrictions on sideloading of books FWIW.

I have an older Nook, but I found it too big to reliably take with me and use.

I just recently got an xteink x4 and flashed Crosspoint firmware on it. It's tiny and portable, and actually fits in my purse or tiny pockets! It'll take epub files loaded onto its SD card, and just sorta works.

I have a pocketbook. It is comparable to an amazon device in basically every regard, but it does not spy on you or show ads.

10/10 would buy again. And again. And again. Because I keep losing them.

The brand Uelshitama. It's great.

I have a Nook. Pretty buggy device, its ony real killer feature is that it lets me buy books other than from Amazon.