Purely by the looks of it, Colt uses the best synthetics for their handguards. I cant say I know how or why, but by the look and feel, they appear to be the best. However, they do use double heat sheilds. I rather preffer the heat to escape the barrel rather than being trapped in by double heat shields.
I have a couple pair of CavArms handguards that I have been very happy with so far. Id suggest theirs or Rock River Arms.