Nothing beats watching a woman masturbate, my favourite sexual fantasy. I have been fortunate that several of my relationships over the years have been with ladies who were only too happy to oblige my biggest desire. One in particular asked me very early in our relationship "what are you fantasies" and I replied watching a woman masturbate while sunbathing on a hot sunny day.
A few days later she came to stay overnight and the next morning had a bath, and while I was preparing some breakfast made herself comfortable on my front lawn stark naked. After drinking a cup of coffee she said would she like me to fulfil my fantasy and asked me to go back into the house and watch from the bedroom window. After getting to my viewing point she had moved the towel to the middle of the lawn and was laying on her back playing with her clitoris, she then opened her legs and proceeded to insert her fingers into her vagina and then began thrusting her hips up off the towel. Needless to say my property was pretty isolated and had quite a concealed garden, however had the postman arrived that morning he would have one hell of a surprise.
To say it was an amazing display would be an understatement, but this lady was incredibly sexual and taught me many things about women in general.