That's the question. I know about gamma ray bursts, is there anything more energetic?
Wikipedia has a page on this:
I would have guessed the recently observed merging of 2 black holes, which, while just hard to detect gravity waves, in terms of energy it was about the equivalent of the entire rest mass of our sun. Two large (but not super-massive) black holes each about 30 solar masses, merged and released about 3 solar masses in energy in gravity waves. That's a huge amount of energy.
I'd have guessed wrong though, as an observed gamma ray burst was bigger. (see link above for precise estimates).
In theory, since I assume you're talking about localized events, when 2 galaxies merge and their super-massive black holes collide, that's probably, hands down, the most energetic local event, far surpassing the largest gamma-ray bursts. Gravity waves might not be what people think of as energetic, but they can be enormously so. If you're talking more typical "bright flash", photons and accelerated particles, then the winner would be the largest gamma ray bursts. Hypernova's are up there, but not as large as the largest Gamma-ray Bursts.
Now if you want to get flexible on what "local" means. The gravitational energy of two large galaxies falling towards each other might get you a larger number, but I'm a little fuzzy on whether that qualifies as a local event", which I've built this answer on.
It depends on the gamma ray burst, but it is possible for some quasars and some high end super/hypernovae to produce an equal or greater amount of energy (this is from the perspective of those that involve single stars or stellar remnants-we are leaving galaxies, galaxy clusters, etc. out of it)