If you read epubs frequently, just on your pc: foliate
If you read epubs frequently, and want to also manage your ereader library: calibre
If you read epubs occasionally and want the same software for reading everything: okular

There's calibre, and then there's everything else. Also take a look koreader. You can also use koreader from calibre

pandoc file.epub -t plain > file.txt
less file.txt

I really like Foliate. Great interface, easy to use, enough customization. It's available through Flatpak.

If you want something more feature rich and don't care too much about UI, then Calibre.

Calibre's ereader is quite good.

Firefox with some extension when in a pinch.

Otherwise, Calibre.

I just use Foliate but on my current system Foliate has a weird bug for me that it cannot open any books for some reason so I'm using Readest now.

For actual reading on linux I use fbreader. For eBook management I use calibre.

But 95% of my reading is actually on old eInk devices (kindle, nook).

This is a tough one. All readers I tried had something that annoyed me. If forced to read on desktop, I will use Calibre. But nowadays I prefer calibre-web to put the epub automatically on my ereader.

Emacs, of course.

I mostly use Foliate, but I actually do most of my ebook reading on my phone.

Readest and Foliate are the best ones

I use foliate

I like KDE Arianna.
Unfortunately, while in the past (I think version 24) it would show 2 columns in portrait mode, now it only shows 1 page in the middle, which I personally consider a regression.

Aside from that there've been some improvements. While in the past I had to flip pages by pressing arrow buttons at the bottom, it now supports swiping the pages, which is much nicer on a touchscreen.

KOReader is goated. So many options, I use use it mainly on phones but it works on PC as well.

Sioyek.

The greatest e-book reader

I use Calibre. I've tried others but many of them are not very customizable.

Foliete FTW

It is web based, but I really enjoy using the flow epub reader.

I use Zathura. Not really meant to be an epub reader (ie missing ways to change the font)—afaik it just converts it to a pdf then displays the epub using the pdf engine. But it works fine for my purposes and means I can use my preferred pdf reader for epubs too.

I only read on mobile (android) using Readest but they do have desktop versions and I assume they're just as good as the mobile version.

They just added functionality to play audio overlays (like from Storyteller), too, which is something I've wanted for a while.

I use baca (because i3) https://github.com/wustho/baca

I like https://terminaltrove.com/lue/ although it's for listening

Calibreweb

I use epy from the command line. It's available in the official Void repo, or in the AUR in Arch.

https://github.com/wustho/epy

Foliate is lovely on my tablet, and OK on my laptop.