I came across KOReader when I was trying to jailbreak my kindle. It's UI looked great on e-ink screen. And it handled almost all ebook formats properly.
Lately, I've used it on Android, and UI which is more suited for e-ink screens, look not so polished on phones, but that's just nitpicking. It's fully usable and keeps adding support for new platforms.
It really shines on e-ink Android readers such as the tablets Boox makes. I almost exclusively use it on my Boox because the built-in reading app is terrible.