Odinworks - I have the sport forend, but they make the kmod type also.
Seekins MCSR V2 - this has the keymod system and is a little fatter than the Odinworks, but that allows my suppressor to fit inside.
I really like both, modular rails allow addition of bipods, mall ninja stuff just about wherever you want to put it.
I don't like the Midwest Industries type locking system, sometimes the part where the two screws tighten the rail to the barrel nut gets in the way of where I want to grab it and it can get irritating.