SM scouts

None of the legendary chapters are particularly famous for using scouts. The Dark Angel and the Space Wolves have scouts that are well known but not as snipers.

My advice would be to make a trait army, that way you don't need to worry about finding an legendary chapter which fits. I'm currently making such an army, here's the list just in case you're interested... http://www.librarium-online.com/forums/showthread.php?t=49970

The Raven Guard relies heavily on Scouts, and their ability to infiltrate in front of the main army and set up attacks - they rely on them because of their survivability for extended amounts of time behind enemy lines.

They also prefer things like Drop Pod Assaults and Assault Squads.