What do you mean which one? It's /bin/sh. /bin/sh has always been the shell used, because it's the only one guaranteed to be on all unixen. No system ships their init scripts written in ash, csh, tcsh, ksh, or anything besides /bin/sh (except Linux, which may have used /bin/bash, since /bin/sh was symlinked to /bin/bash)
You might have made your personal init scripts in some other shell (until you learned about design failures of csh), but I'm willing to bet your Unix distributor did not.