I would have to say the British Fairbairn-Sykes Fighting Knife
Though known as the FS Fighting knife, this was not designed to be a knife fighting knife, but primarily designed to be used in silent killing actions such as sentry take-outs. The techniques of effective use were taught to various special forces at Highland training centres such as Lochailort Special Training Centre (STC) and Achnacarry, which was the Commando Basic Training Centre (CBTC) from 1942-1945
http://www.olive-drab.com/od_other_edged_sykes_fairbairn.php