Well nvim, obviously. It's pretty much fully featured. With LSP plugins you get all the code completion, hints, type info, docs and so on. You also get typical navigation like 'go to declaration' and some basic refactoring. And all inside the best editor there is. I'm using it for C, JS, JSX and Rust and all works great. I honestly prefer it to IntelliJ, it loads faster and is more responsive.
My favorite is Kate because it's less of an IDE and more of a text editor with side panes for the project tree and a terminal to run the program. Easy enough to set up a hot key to save-build-run. I think that's all I need?
Lots of replies mentioning Emacs but Emacs out of the box is gonna be essentially a text editor (insert obligatory: Emacs isn't a text editor; it's a LISP interpreter).
However, install Doom Emacs, and you have a full IDE experience for essentially any language you could ask for. I highly recommend it.
Helix is pretty slick, but it’s not very extensible. Very easy to use and if the out of the box features are good enough for you then it’s a fine IDE.
Neovim is my preference unless I’m working with Jupyter notebooks, in which case I switch to vscodium. It’s a pain in the ass to set up. I took the easy way out with LazyVim. It’s fast to work with and I can use it for almost everything.
I dabbled with emacs many years ago. It’s like vim but completely different. You can make it do anything. Personally, I don’t care for the keyboard shortcuts. It’s probably easier to pick up than vim, but all the key chords and sequences are too much for me. In any case, anyone willing to look at vim should also take a look at emacs.
VSCodium is accessible and extensible. You can’t go wrong with this one. It can’t refactor like the Jetbrains stuff, but if there’s anything else it can’t do then I don’t know what it is. It’s a great IDE.
Really, any of these can do just about any job and do it very well. There’s no choice that clearly stands above the others. It really comes down to personal preference.
I used to like MonoDevelop maybe 10 years ago, but it's not around any more. If I remember correctly, it was the only open-source IDE that supported C# and ran on Linux. That was before C# and .NET were open-source and Mono was the only way to run C# apps on Linux. Things are way different now.
The best today is obviously nano. It has syntax highlighting, auto-indentation, and at some point they made it so Ctrl+S saves the file. What more do you need? (cut and paste still use weird shortcuts though)