Mont le Brevent
Mont le Brevent is a very nice run, easy accesible with the lift. You can pick your starting point where you like, depending on which of the upper couloirs you want to fly.
If the Start right at the top is too much for you, you can ski down around the peak and start there. Please watch out for paragliders when using the official landingplace.
Please visit this page for Chamonix rules: http://www.chamonix.net/english/winter-activities/speed-riding