CloudFlare having that outage a while back was bigger, I'd say. GitHub is only used by a small percentage of people out there.

If you want to go by "percentage of the Internet taken down", the biggest outage ever has probably been the Morris Worm, since a lot of major insitutions quarantined themselves by closing down their network links to other sites until they could identify the problem, purge it, and secure their systems against reinfection.

https://en.wikipedia.org/wiki/Morris_worm

The Morris worm or Internet worm of November 2, 1988, was a malicious self-replicating computer program that affected VAX computers and SUN-3 workstations running the 4.2 and 4.3 Berkeley UNIX code.[1] It is one of the oldest computer worms distributed via the Internet, and the first to gain significant mainstream media attention.

The Internet was partitioned for several days, as regional networks disconnected from the NSFNet backbone and from each other to prevent recontamination while cleaning their own networks.

The Morris worm has sometimes been referred to as the "Great Worm", named after the devastating "Great Worms" of Tolkien. The Morris worm had a devastating effect on the Internet at that time, both in overall system downtime and in psychological impact on the perception of security and reliability of the Internet.[17]