What's the saddest episode of a show?

This is geeky, but the last show that hit me in the feels was Futurama, where the main character who accidentally time travelled tries to clone his beloved dog from the past. He ultimately decides this is not the right thing to do and comes to peace with losing the dog, but at the very end of episode it's revealed the dog waited for its owner till it's dying days, for nothing. The main character never finds out

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