Who's the first person you think of when you're masturbating?
These days, usually one of my characters. In the past, though there have been a few actors/actresses, e.g. I was hot and bothered for the women (Maren Jensen, Laurette Spang) on the original Battlestar Galactica, which hit at just the right time for me. And I had a longstanding crush on Brooke Shields in my teens and twenties (we are the same age) so she came up in my fantasies.
There have also been women I know in real life, such as classmates in high school and university and later co-workers but I have kind of stopped doing that. I work with several hot women that I have mild crushes on right now and none have figured in my fantasies. Not sure why. The very yummy looking young* financial analyst in accounting who I have trouble keeping my eyes off of, for instance. Not sure why I have never stroked one out to her.
*For certain values of "young". She has a master's and her daughter is like 9 or 10 so she must be 30ish at least.
Learning to draw the human body becomes a lesson of another kind.
The woman by the pool shared her art ... and so much more.
Lucy's Body of Art - Wherefore Art Thou comp entry