The standard cron that nearly everyone used to use would have been Williamson cron. Paul Vixie's PD cron (which was the original name for it) was a clone from 1987, and it of course was not used where people used the standard one, so was clearly not the one "nearly everyone" used.
It never became it, either. It was fairly quickly forked. Several times. Debian cron was forked from PD cron in 1994 by Ian Jackson, for example. The "standard one that everyone used to use" but only in the limited field of Debian Linux (and no longer, nowadays with Debian Linux version 9) was that fork, which M. Jackson actually named "Debian/GNU Linux's". RedHat made a comparatively much more recent fork. OpenBSD and FreeBSD have their own (different from each other) forks of long standing, too.
Mike Meyer apparently made a GNU cron for the Free Software Foundation in 1987. If it indeed ever existed in the first place, it has since disappeared without a trace.
I provide pre-made service bundles for the nosh toolset for various crons, and I have so far had to make separate ones for Dillon crond, Debian cron, Vixie (a.k.a. ISC or PD) cron, Guenter bcron-{spool,start,update}, Godouet fcron, and OpenBSD cron; because they are significantly different from one another.
In part, this is because although the crontab utility is actually standardized, in the SUS, cron (like many such non-ordinary-user-facing internals) is not standardized. This is actually a good thing, inasmuch as the radically different multiple untrusting dæmons architecture of Guenter bcron would not have been possible if it had had to conform to a standard of one monolithic cron dæmon.
The history of cron is indeed fascinating. But I think you interpreted the word "standard" too literally. I meant "the most commonly-encountered and well-known cron implementation to date."
It never became it, either. It was fairly quickly forked. Several times. Debian cron was forked from PD cron in 1994 by Ian Jackson, for example. The "standard one that everyone used to use" but only in the limited field of Debian Linux (and no longer, nowadays with Debian Linux version 9) was that fork, which M. Jackson actually named "Debian/GNU Linux's". RedHat made a comparatively much more recent fork. OpenBSD and FreeBSD have their own (different from each other) forks of long standing, too.
Mike Meyer apparently made a GNU cron for the Free Software Foundation in 1987. If it indeed ever existed in the first place, it has since disappeared without a trace.
I provide pre-made service bundles for the nosh toolset for various crons, and I have so far had to make separate ones for Dillon crond, Debian cron, Vixie (a.k.a. ISC or PD) cron, Guenter bcron-{spool,start,update}, Godouet fcron, and OpenBSD cron; because they are significantly different from one another.
In part, this is because although the crontab utility is actually standardized, in the SUS, cron (like many such non-ordinary-user-facing internals) is not standardized. This is actually a good thing, inasmuch as the radically different multiple untrusting dæmons architecture of Guenter bcron would not have been possible if it had had to conform to a standard of one monolithic cron dæmon.