The best response would be heartfelt and sincere, tailored to your personal relationship and feelings for each other. Here's an example:
"I promise, I'll be by your side through all the ups and downs. We're in this together, now and always. I love you, and I won't let you face anything alone."
Keep in mind that what might sound romantic to one person might not resonate with another, so be sure to communicate in a way that feels authentic to you and your relationship. The most romantic things often come from the heart, rather than a script.