That would be Gael Monfils. No one has won the junior slam since Edberg in '83, I think. I think it is widely recognized that junior slam success is no guarantee for ATP success, but obviously if a kid can work to improve much more after junior success, then he can follow it up favorably. As a side note, Federer is one of only a few people to win both the Wimbledon boy's title and gentleman's.
Proudly donning the Nadal avatar until Fed either wins Roland Garros, or serves Nadal a bagel.