My opinion as a female is that if a man I had never met walked straight up to me and told me I was pretty I would politely thank him and then promptly find a way to extract myself from his presence. It causes me to feel very uncomfortable. I don't care for small talk (as I don't really know how to do it well myself) but in polite society, it is necessary. Also, and in the context of small talk, there are more subtle ways to compliment a woman's beauty. Tell her she has beautiful hair or that you like her earrings. Something simple such as that. For example, I have red, naturally curly hair and am quite used to strangers of all sexes commenting on it. I've had men walk straight up to me and tell me they thought my hair was beautiful and I don't think that's strange at all.