The release of 13. After years of false starts and wondering if they could still pull it off they announced they were reuniting to make an album. The production was marred by Tony Iommi's illness and the dispute with Bill Ward and the band members themselves weren't fully convinced they could pull it off. Then it came out and for the first time in their career they were met with critical acclaim. And more importantly, it got a hugely enthusiastic response from the fans and instantly went to #1 in fifty countries.
Not to mention that the album ends with "Dear Father" using the same sound of rain, thunder, and church bells that began their debut album. A lot of longtime Sabbath fans probably got goosebumps when they heard that.