Depends on device for me. For android I use Librera for books, Tachiyomi/Kotatsu for manga/comics, on the old Kindle I was gifted (Kindle Touch 2) I use KOReader so I can read epubs. For desktop I do use Calibre for reading, though I'm not a big fan of their reader. I mainly read textbooks on desktop and find the search features useful, which is the main reason for using it, it all works well enough. I had issues getting Okular to work well on my computer, but I've heard it's good? Here's hoping I can unify things a bit in the future.

Eta: I forgot I actually started using Seeneva for comics, since I like the speech-bubble zoom feature

I use Koreader. It's a bit heavy because it has basically any option you can think of, it works on a wide multitude of platforms, and can have weird bugs sometimes but hardly ever. But the way I read, I need some settings that aren't in very many readers and it has them. Try it out.

ReadEra on android and Calibre's reader on Xubuntu desktop.

Might look into some of the stuff suggested here.

Librera is the one I'd recommend if you care about customisation, and it also has TTS (Text To Speech), which is why I use it almost exclusively.

Myne is a beautiful and minimalistic app that lets you read books from the Gutenberg project. I have asked the developer, and they have no intention to add TTS functionality to the app.

Shosetsu is the best app for fanfiction (AO3 extension) and lightnovels. It recently got a TTS feature as well, but that feature is not very usable in its current implementation.

Foliate on PC, Librera FD on Android.

koreader is good if your platform supports it.

I use a second-hand kindle on airplane mode and just transfer books to it from calibre. E-ink is better for my eyes.

Believe it or not, DeGoogled-Chromium with Dark Reader. It makes for perfect dark mode

I really like Alexandria on desktop and there's android support on the roadmap on GitHub.

If I only read on my phone, I'd use Librera. But for syncing between my Onyx Boox device and my phone, KOReader is hard to beat.

I run Calibre-web tied into my Calibre server so I can read on every device I own.

Book reader on android.

I also use Calibre as a library manager, but I haven't found another way to read that is not my kobo. The not e-ink screens just tire my eyes too much.

I use Lithium on android. The dark mode is great for night reading.

Am I just a tech-peasant by using Book Reader? It's not the flashiest but I dig it.

I use Ttsu Reader. Browser-based so it'll run on anything, and has all the conveniences thereof.

Edit: Unless I'm actually using my Kobo, then I'll just use my Kobo.

I am surprised I haven't seen Kavita show up here yet! It's a very nice self hosted ebook/manga/comic reading server that you can access from a web browser on any device. It's really matured over the years.

I have a few thousand novels in there and really like being able to hop between devices while keeping my spot in a book.

Okular it can read pretty much any format. MoonReader+ on my android devices.

Foliate has been absolutely amazing. I love the simple interface that doesn't look as dated as calibre.

I have a cheap Kobo and put KOReader and Syncthing on it.

Try foliate or koodo reader, they got a clean UI

Only thing I miss from Apple was their mobile eReader. Anyone know of a decent copy of that one?

I'm holding out for the PineNote from Pine64.

I use my hacked kindle 4 and Bookreader on android tablet.

Remarkable 2, but the PineNote might be usable now.