Gamma Ray Burst. It's on the order of 10^45 J. source
"preemptive killing of cops might not be such a bad idea from a personal saftey[sic] standpoint..." --Keevan Colton
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Gamma ray burst described: "million million million" times a 1 MT bomb, more energy than a star produces in its lifetime.
(1E6)^3 = 1E18 MT
However, their "number" isn't exactly very precise. According to a more reliable site, a GRB can release on the order of 1E45 or even up to 1E47 Joules
DS blast: 1E22 MT or 1E38 Joules, lower limit.
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In terms of raw power, a gamma ray burst. However, only a small portion of that burst would ever hit a planet - best case would be that the biosphere and a good chunk of the crust were roasted off the planet. With the Death Star, all that energy is focused towards the planet, resulting in a nice big boom.