The elevator pitch that got me most excited about rationality is from Raising the Sanity Waterline. It only deals with epistemic rationality, which is an issue, and it, admittedly, is best fit towards people who belong to a sanity-focused minority, like atheism or something political. It was phrased with regard to religion originally, so I'll keep it this way here, but it can easily be tailored.

"What is rationality?"

Imagine you're teaching a class to deluded religious people, and you want to get them to change their mind and become atheists, but you absolutely cannot talk about religion in any way. What would you do? You'd have to go deeper than talking about religion itself. You'd have to teach your students how to think clearly and actually reevaluate their beliefs. That's (epistemic) rationality.

"Why is rationality important? Shouldn't we focus on religion first?"

By focusing on rationality itself you not only can approach religion in a non-threatening way, but you can also align yourself with other sane people who may care about economics or politics or medicine. By working together you can get their support, even though they may not care about atheism per se.