What is the best thing to say to a veteran?

What is the most appropriate and respectful way to approach a conversation with a veteran? Should one express gratitude for their service, inquire about their experiences, or simply acknowledge their sacrifice in a subtle manner? Is there a universal phrase or sentiment that resonates deeply with veterans across different backgrounds and eras? How can one ensure that their words convey genuine admiration and empathy, while also respecting their privacy and boundaries?