I have to vote mechanical catastrophe is worst. Almost all pests have workable treatment — and they feel more “part of the game” to me.
A broken tank or electrical fire is the ultimate nightmare for sure. Broken heater or carx disaster could nuke the tank, but at least you can mostly avoid with redundancies. And it won’t kill your house too.
Diseases next bad, especially some horrible ones like uronema, velvet, brown jelly (though antibiotics might be a silver bullet so move that down), Ich
Worst pests:
1. Dinos
2. Tied AEFW and MEN
3. Invasive corals you really knew were a dumb idea but went for anyway
4. Algae like brown wire, cladophoropsis, etc that can’t be starved and nothing small eats. (But H2O2 eats it all!!)