What are the top visited web sites, i.e., those with the greatest reach?

The website ClickZ aggregates web usage marketing information especially in their Data - Date Insights section. You can also search "top visited".

The eMarketer database will also have this information.

Also look at the Internet Advertising Bureau, which often publishes this information in "Insights and Research" tab section.

What percentage of internet advertising is banner advertising?

In the eMarketer database, click on "advertising" under the "Topics" tab, then "online advertising". Type in the term "banner" in the terms and fliters form.