Actually it was Yggdrasil. First Yggdrasil distro was 1992, first Slackware was 1993.
<proud Slackware user to this day> :D
(edit) Actually I am wrong. I just checked my dates, SLS came out in Sep 92, Yggdrasil in Dec 92. In early 1993, SLS split into Debian and Slackware. And the rest is history...