I live in Prague and work every day in the center.

By far the worst are the Brits who get fall down drunk and try to start fights. Americans are only annoying in that they talk loudly and are often kind of dumb. But they are generally polite. They don't hold a candle to the Brits. The rest are generally mild enough as to not be noticeable unless they are in a big group blocking the sidewalk which can be any nationality.