I hate to be the bubble burster here, but $850 for just the rifle or $850 for your total budget? I tried to stay in the $750 range tops, but that was just for the rifle (M&P 15 OR). But, with that particular model, you have to add your own aiming device: optic (red dot, EO, etc.) or BUIS's or both like I did or a carry handle. That adds a bit more to your overall budget. Even if you get a flat top of some kind, you need to get some kind aiming device.
"Top five"? I'm guessing your thinking about top five name brands (colt, bushmaster, rra, armalite, stag). If you want a name brand, do some seaching and try to part it out, upper + lower, or part kit. Thats really the only way to stay under $850. A bit more work on your part, but you will save some $$$$. I got my M&P from an off site sponser for $757 ($50 Smith rebate + $8 insurance). I havent used it yet and I do not know anyone who has used one, so I will not include the M&P as a Top Five rifle, based on bias and lack of perfromance knowledge.
On second thought, TOP FIVE, do you mean just the rifle itself? If you build in terms of uppers and lowers, your talking makers like sun devil, mega, cmmg, noveske, lmt. Perhaps they're more of a custom shop but definiately top quality equipment.