Nuzlockers: What Pokémon death hit you the hardest and how did it happen?

While I was playing a Pokémon Yellow Randomized Nuzlocke, Leviathan, my Gyarados (my favourite Pokemon) from Route 2 died in Cinnabar mansion.

I was set to two shot a Clefable with Earthquake until it hit a Metronome Thunderpunch that ended up being a Critical Hit. Talk about the worst of luck.