Well, that depends on the context. Billboard is *the* chart for the U.S. If they talk about a #1 hit, they will discuss on which chart. For example, they do make the distinction between country charts, modern rock charts, adult charts, etc.
That said, the Hot 100 is the primary chart. If a song is #1 there, then it is indeed the biggest song in the country that particular week.
Sadly, U2's "Vertigo" missed out at its chance to be another Top 10 hit (maybe higher) for U2. Billboard continuously changes how they track the top songs. This is both good and bad. Good because changing times mean tracking techniques also have to change. For example, in the 50's, Billboard tracked how many times a song was played on jukeboxes! Clearly this type of tracking won't mean much in today's world. However, the bad is that by constantly changing, there's no way to normalize the data. As I wrote, "Vertigo" easily would've been another Top 10 hit for U2 had Billboard tracked legal online sales. Billboard started doing this about 2 months after "Vertigo" dropped on the charts. As a result, "Vertigo" peaked at #31 based on airplay alone. But add in the 300,000+ copies sold on iTunes alone, and this probably produced a Top 10 hit! Indeed, when Billboard started adding in legal downloads, "Vertigo" jumped almost 20 places on the charts! But by then, it was too late.
Of course, the U.S. isn't the world (despite what Bush may think). In this forum, we post chart news around the world. U2 may not hit the top in the U.S., but often they will top the U.K. or Irish or Dutch or Canadian charts. ABoY, for example, wasn't a big hit at all in the U.S., not even cracking the Top 5 on the Modern Rock charts. However, the CD went to #1 in Canada and the song broke the Top 10 on airplay (I don't know the highest position - I do know it hit #7 at least). So that makes it a pretty big hit. In the U.K., "Sometimes..." hit #1 on the sales chart, which seems to be the big chart in the U.K. CD singles are still strong sellers there, which contrasts most places around the world (and is one reason why U2 has no CD singles in the U.S. - barring a few token exceptions, CD singles in the U.S. have horrid sales).
There is also a world chart, which takes into account all information. I think "Vertigo" did top the world charts, even though it "only" peaked at #31 in the U.S. (thanks to Billboard NOT monitoring legal download sales at the time).
So it's up to you which is important and what data you are seeking.
As for the U.S., on the Hot 100, U2 have only had two #1 hits, both from the JT era - "With or Without You" and "I Still Haven't Found...". But they have had many more #1 hits on the U.S. Modern Rock and Mainstream Rock charts, including "Vertigo".
