Preferably all of them, but that is up to you!
Most web sites are designed for the smallest screen resolution, which is in the mean time considered to be 800x600 pixels. The most common resolution is currently 1024x768 pixels.
Only if you say that you know that your audience is 'more advanced' in that sense and uses larger screen resolutions, we will create and design accordingly.
Additionally, there is the option to create a website using a liquid layout, meaning it adjusts to the user's screen resolution. It nicely expands on larger resolutions. The limitation of this option is that text formatting, image alignment etc. has to be flexible/expandable, and as a result such websites cannot have elaborate layouts in the main content area.