Porn genres deviate from reality as often as they get it right, 22-year-old MILFS, 38-year-old barely legals, etc., but big deal, porn is not reality so it's all fair.
Most of my favorites would be GILF's and probably great grannies by this point, at least old enough to be great grandmas, like Ginger Lynn who is on my top ten all-time list.
Right now, I'd give Brandy Taylor top billing as a MILF, at least in the top ten, but damn there's so many to choose from, and they cross genres so often it gets confusing anyway, you can't distinguish between a MILF and a BBW or FFW when the babe is both, so it's hard to say definitively when you consider all factors, my 2 ¢.