It depends. Sir can have different meanings depending on context.
If used as a polite way of addressing a man ; 'meneer'
If used to address a male teacher; 'meneer' (in highschool) 'meester' (in elementary school)
At the beginning of a letter ((Dear) Sir,); (Geachte) 'heer'
A title used before the first name of a knight or baronet; I don't think we would actually translate it here, since it is an English title. Sir Winston Churchill remains Sir Winston Churchill. Sir Launcelot is called 'ridder' Lancelot in Dutch, but I don't really think this is the same thing.
Maybe if you were a bit more specific about the context I could be of more help.