which is best website structure in term of SEO & Usability ?

Can you tell me that which is best website structure in term of SEO & Usability ?

I can say that you should make sure any site is xhtml compliant and that pages are no more than 3 clicks deep.

There are also some other aspects that are ensure title tag is first line of sode after the head tag, use headings to seperate content, try to use hyper text links over nav buttons or drop down menus, etc.