How is one's "worst enemy" determined?

[Fistleaf]

Is it due to razing of cities or very negative relationship score?

[Quillan]

At default, it's the nation that has the lowest total diplomacy modifier. I don't know how the AI breaks ties. It's also subject to change; one civ might declare war or refuse a few "help" requests and get enough negatives to drop them below the standing of the old worst enemy.

[Fleme]

The buggers still always seem to have open borders with their worst enemies so when I think I'm safe from an attack from AI X through AI Y because they're worst enemies I get attacked by X on the front where I least expected for it to happen.

[johnmcd]

I had a beaut where someone asked me to help them in a war, which suited me so I did.

Then immediately got a -1 'You declared war on our friend' modifier, wtf?