WordPress provides following permalink structures

Default ( /?p=123 )
Day and name

Month and name

Numeric

Custom Structure

/%category%/%postname%/

%monthnum%/%postname%.html

%postname%.html

%postname%

(common custom structures)

Default ( /?p=123 ) structure is not seo friendly because it gives no information about post content .

Best known permalink structure is /%year%/%monthnum%/%postname%.html and it is most prevalent .I also use this permalink structure .

Other effective perma structure and used by many famous blogs is %postname%.html