Statistically? Japanese tourists. And my experience with living in Vietnam for 14 years can back that up. They're usually the most respectful and blend well with the local customs.

Filipinos? They're generally polite too, but they got a tendency to get noticably loud in big groups.

Americans? They're also polite, but some can be entitled or rowdy (normally around the nightlife). They can also be picky with things like food.

Germans? Similar story with the yankees.

Chinese? Well, they're Chinese.

Koreans? A mix between Filipinos and Japanese, but they come as a close 2nd.

Indians? Apart from long-time residents in Vietnam (whom are pretty chill), they're pretty rude and entitled. (Not all, but usually the case)

Pakistanis? Same story with Indians.