I highly recommend LWRC, for anyone who wants the cool parts of a piston system but want to stay familiar with the AR-15 series. Their most basic model is a vanilla M4 style, their more advanced ones is all tactical rails and magpul stocks for anyone who is into that. You can get an upper ordered if you already got a lower, and they're putting cold hammer forged barrels for all rifles now.
In addition, they got some big hitters on their team. One of them is Major General (retired) Gary Harrell, who among a lot of accomplishments, was the Delta Force squadron commander during the Black Hawk Down event. Retired Master Sergeant Paul Howe (he posted here on AW a few times) is also endorsing LWRC as well, and if I remember from reading the Ar-15.com forum, is working with them to create an official Combat Shooting and Tactics carbine built to his specifications.