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).

The big problem with ksh93 is that it is not maintained.

After David Korn retired, an attempt was made to bring out a new version which failed dramatically, and AT&T reverted to Korn's final release.

SmartOS uses ksh93 as root's shell.