RE: Not an assault rifle!
Unless you're willing to wait, you're likely to be stuck with whatever is in the dealers' racks.
Most of what I've seen lately have been Bushmasters, with DPMS a close second. From what little I know about them, either of them would be a reliable rifle. I've also seen a few of the S&Ws, and know a couple owners who are quite happy with them.
If you want a Rock River, plan on waiting - and the wait is worth it. I have a Rock River Entry Tactical and a Colt Gov't Model AR15A2, and the Rock River is the better shooter, hands-down - though the Colt's just as accurate as any military M16A2 I've used.