That statement ("Ii onna") is all that is required. Ii has a vague meaning like many other words. It can mean ok or beautiful or perfect etc. In this context, "beautiful woman" is a good translation. She may ignore you, but you might get a blush or a smile or an invitation to coffee.
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.