i dont know about the best under 15 part but best 12 year old in the ll world series that yea.
shit i got a cousin in whittier california that is 15, 6 foot 1inch tall, 170lbs, and can throw high 80's. he was a freshman starter for the varsity team Whittier high school in east LA this past year.
and im pretty sure their are others around the country but i do think that osbourne will dominate against his own age group up until he gets drafted (probly outta high school).