I particularly like ksh93. It's a virtual machine, which makes it super fast for a lot of scenarios.

The ksh family is my personal favorite, in particular mksh and ksh93. Both with excelent feature sets. I even made a platformer game that works on them https://github.com/alganet/tuish/blob/main/examples/game.sh (and zsh/bash/busybox too) to show how feature-complete they are.

Shell interpreters are such a broad subject, I could go all day talking about cool things that can be done with them.

I want to get people focusing less on the spec. It's for whoever implements interpreters, not people who write scripts. And there's a gap on that, which I'm trying to cover (with full ksh support, much more than you think).