If I were put in that position, I think I'd try and open a more tactical dialogue than a chance to spout off my own preferences:
Alien: "What proof can you offer that your species should be spared [the plan]?"
Me: "There is many things on Earth. What do you and your kind value?"
Alien: "[Answer]"
Me: "[Does my best to showcase relevant examples in a positive light]"
...
Maybe I'm taking this question to literally if it's more a metaphor to frame "what's good in humans?", but facing the situation as described, I would (were I able to keep my head) approach it as a diplomat and prospector.
Alien: "What proof can you offer that your species should be spared [the plan]?"
Me: "There is many things on Earth. What do you and your kind value?"
Alien: "[Answer]"
Me: "[Does my best to showcase relevant examples in a positive light]"
...
Maybe I'm taking this question to literally if it's more a metaphor to frame "what's good in humans?", but facing the situation as described, I would (were I able to keep my head) approach it as a diplomat and prospector.