The choice between raw or caramelized onions on a burger depends on your personal taste preference. Raw onions provide a sharp, crisp bite that can contrast nicely with the rich, juicy flavors of the burger. They are typically thinly sliced or diced.
Caramelized onions, on the other hand, offer a sweet, savory depth of flavor. The process of caramelizing onions draws out their natural sugars and enhances their sweetness, creating a delightful contrast with the savory components of a burger.
Both are excellent choices and can add different layers of flavor to your burger, so it comes down to what flavor profile you're in the mood for. You could even try a combination of both for a more complex flavor experience.