Tsar Bomba: World's Largest Nuke.

The world’s largest nuclear weapon ever constructed and detonated was the Soviet’s Tsar Bomba [wiki]:

The fireball touched the ground, reached nearly as high as the altitude of the release plane, and was seen 1,000 km away. The heat could have caused third degree burns at a distance of 100 km. The subsequent mushroom cloud was about 60 km high and 30–40 km wide. The explosion could be seen and felt in Finland, even breaking windows there. Atmospheric focusing caused blast damage up to 1,000 km away. The seismic shock created by the detonation was measurable even on its third passage around the earth.

Since 50 Mt is 2.1×1017 joules, the average power produced during the entire fission-fusion process, lasting around 3.9×10-8 seconds or 39 nanoseconds, was a power of about 5.3×1024 watts or 5.3 yottawatts. This is equivalent to approximately 1% of the energy output of the Sun during the same fraction of a second. The detonation of Tsar Bomba therefore qualifies, even to this day, as being the single most powerful device ever utilized throughout the history of humanity.

Link [YouTube] via MilitaryFix