Last year about this time I was agonizing over which evil black rifle to buy and finally settled on the Stag Arms 3G rifle. I love the Samson Evolution handguard and rifle length gas system, not to mention the incredibly sweet Geissele trigger!
Most any of 'em will get the job done, but no sense in buying one and then forking out good money to change stuff on a factory rifle. Better off buying it like you want it to begin with, or build one to suit your tastes