The ideal width is all of them. The magic word is Responsive.
Not necessarily true depending on the nature of the site. While responsive designs are ideal, a maximum width is a good idea. If you have a site with a lot of text, like this one, a blog or similar, a paragraph becomes a very, very, very long sentence on wider screens and difficult to read, not to mention it looks rather stupid. Personally, I max out main content areas at 800px which is still a good guideline.