I think destructive is the wrong term for this, but meh. Here's what I think:
1st Place: Gravity itself
2nd Place: The Big Crunch (opposite of the big bang; a gravitational collapse of the universe into a singularity)
3rd Place: Black holes (the cumulative gravitational effects are likely more damaging than any single explosion)
4th Place: Hypernovae
5th Place: Supernovae
6th Place: Cosmic Rays (higher frequency than gamma rays, btw)
7th Place: Opposable Thumbs

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gg.
While we're on the topic of force; someone name for me the four natural forces in order from strongest to weakest.