The largest flare in modern times was recorded in November 2003 and was estimated to be an X-40. It, too, was on the limb of the Sun and so its full impact was not felt on Earth. That flare was part of an unprecedented series of 10 major flares within two weeks; at least one Earth-orbiting satellite was disabled and one instrument aboard a Mars-orbiting craft was knocked offline.
Forecasters at the NOAA Space Environment Center in Boulder, Colorado, US, continue to observe significant flares on the sun Friday in NOAA sunspot Region 808 (the same old 798). This is the same region that yielded a powerful X-17 flare on Wednesday
Sunspot 798 is now rotating into view over the eastern limb. It has now blasted out two major solar flares: X17-category blast on Sept 7th (1740 UT) and an X5-blast on Sept 8th (2105 UT).