Honestly? Bash. I tried a bunch a few years back and eventually settled back on bash.

Fish was really nice in a lot of ways, but the incompatibilities with normal POSIX workflows threw me off regularly. The tradeoff ended up with me moving off of it.

I liked the extensibility of zsh, except that I found it would get slow with only a few bits from ohmyzsh installed. My terminal did cool things but too slowly for me to find it acceptable.

Dash was the opposite, too feature light for me to be able to use efficiently. It didn’t even have tab completion. I suffered that week.

Bash sits in a middle ground of usability, performance, and extensibility that just works for me. It has enough features to work well out of the box, I can add enough in my bashrc to ease some workflows for myself, and it’s basically instantaneous when I open a terminal or run simple commands.

There’s been some controversy around the governance structure and culture with NixOS that has a number of people unhappy. I’m honestly not sure of the details but it’s ptesumably less about the software than the people.