I am wondering if this a bit too provocative than what the original poster asked for. The word onna is often not exempt from sexual connotation. If you want to use this phrase as a Japanese equivalent of "You are a real cracker," well, provoke away.Ikemen-
Just look at her and slowly say " Ii (eeh) onna". It means attractive woman and is often used when a man sees a pretty woman. The phrase is not vulgar.