OpenBSD’s default public domain kornshell fork on OpenBSD, oksh (portable OpenBSD ksh clone) on Linux/MacOS/Other Unix. It has far fewer extensions than something like Bash (which I consider a positive) while being much faster (tested with hyperfine), and the extensions it does have are all useful (arrays, coprocesses, select, .* not expanding to . or .., pattern blocks, suspending of the whole shell).