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It has been suggested that some if not most elliptical galaxies are the result of two similarly sized spiral galaxies colliding: http://spacetelescope.org/images/potw1148a/

The most energetic event I can think of would be two galactic core sized black holes colliding. Has this been ruled out already as a possible cause of FRBs?

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Yes. The timescale of FRBs is too short to be caused by mergers of supermassive black holes. In general the source of an event cannot be any larger than the time for light to cross the source. In the case of a supermassive black hole binary the light crossing time is of order seconds to minutes, but FRBs last for only a few milliseconds. This limits them to be the size of neutron stars or smaller.

The accretion disks around black holes merging could act like a really massive and short lived particle accelerator. However, that's more likely to spit out a few Oh-My-God particle's than just a radio burst.

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