Pricey, but they work and come in all shapes and sizes: the KAC rails. They're mostly all I use anymore. They make several different kinds, from the older, classic tube rail versions to the new URX versions. I like 'em all so far, but the older ones I've used for ten years and they've never let me down, not even on a .50Beowulf. I really like the rail covers, they feel "natural", since I'm not a fan of holding bare rails. Theirs slide and clip on, and you can cover pretty much every rail you aren't using making it more of a normal handguard. I like that. Certain URX forearms you just bolt on the rails and grip panels where you want 'em, they're otherwise stripped. One URX has rails on it, but also has the front flip sight built in.
I think the army switched over from KAC to Daniel Defense for the rails on their new SPR rifles, FWIW. They also used the carbon fiber PRI tube for awhile too.
I always thought the JP Ent. VTAC looked good, but I never got around to using it.