A subject as vast, complex, and diverse as biology will never have a single killer argument. To look for one would be a fool's errand, and to believe one would be the mark of a fool. A singular killer argument is the sign of a simple question. Something evolution most decidedly is not.
The fact that pretty much every discovery in Biology (especially genetics) has supported the argument for evolution should lend credence to the theory of evolution, and Darwin being right. There is no way a Victorian era gentleman would know about DNA to the level we do now, and yet he was so amazingly right about the things he wrote. Yet, it takes an incredibly complex and in-depth understanding to really understand and see the significance. Something that those who would deny evolution haven't the slightest clue on.
Anyway, just wanted to post this. I also would like to bring your attention to Zack Kopplin and his efforts. You all should support him. And if you regret ably find yourself attempting to discuss evolution with a creationist, I suggest that you arm yourself with as much information as possible. Facts, not Fantasy is one place you can go to start that process, but remember:
“Debating creationists on the topic of evolution is rather like trying to play chess with a pigeon; it knocks the pieces over, craps on the board, and flies back to its flock to claim victory.”
― Scott D. Weitzenhoffer