I always think it is funny when reading about who the Father of MTB was. That being said the first group that started modifying bikes and putting road components on them, sharing ideas and building frames and testing them against each other normally as a group had one place that was considered "A Test" > That was MT Tam. http://www.marinfattire.com/Mt_Tam_Trails.html. Velo Club Tam is without a doubt the springboard for modern day mountain biking. They rode Mt Tam

There are so many trails between San Francisco and LA that choosing one that is the "best" is like asking who is the most beautiful blond in the history of Hollywood actresses or not would start a similar type of debate. Very few can or would argue that Mt Tam is not holy ground nor without merit as a "must do" ride. Mt Tam is steeped (yea so had to throw that word in somewhere) in history and meets your criteria.

While married, old, 600 miles away and fat I very much appreciate the offer to take care of your wife. However she would find me far less interesting than a day to herself with a credit card in downtown San Francisco.