The NR tax distorts our metric of value when it comes to rifles, we all know that. Using the criteria of: semi auto, 5.56mm with AR15 magazine compatibility, optics ready, the options are going to be based on price point.
SU16: The least expensive, has a bad reputation for durability issues. I don't know if that got rectified or not, but a rifle that fails structurally after relatively normal usage has a black cloud over it for good. At least it's lightweight and has standard control placements.
T97: some people like them, they aren't optic ready out of the box and require modifications to reliably use many magazines on the market. Poor control placement, users will have more difficulty manipulating the safety and mag release than the others mentioned. Some have had issues with feeding, quality control is marginal. At least these go for less than 1000 new.
Jard48: I have no idea what this rifle is capable of, but there doesn't seem to be manufacturer support available and there are reliability issues. Inexpensive though.
Ar-180

Pretty capable rifles, great design and good ergonomics. Somewhat expensive for what it is, there aren't as many of these around any more. I'd pick one up for under 1500 used if I was looking for a NR black rifle right now.
MR1: I have one and I like it, but there are lots of areas to improve. It doesn't come with a flash suppressor or even a threaded muzzle. The controls are not where they are on an ar15 style rifle. Sights are low like a mini 14, so any optics designed to cowitness with ar15 lower 1/3 will be too high to get a good cheek weld. Still, I picked mine up for 1200 used and it's reliable and accurate. It also has the best trigger I've felt on a rack grade semi, bar none.
M17s: I don't have the K&M version, so I can't speak to that. It's a sleek but deceivingly heavy bullpup. Great control layout, it shoots well and is reliable (pretty much an ar18 action). The stock rail makes a crappy and limited scope mount, get some aftermarket rails to make it a flat top. It's kinda picky with magazines, but likes some of the more common options. They seem to be going for about 1500 used right now.
XCR: Everybody and their mother has shot one or owns one. Nice features, good controls, calibre conversion ready... I don't know anybody that has had the purported loctite and QC issues I've heard so much about on cgn. I find them to be pretty heavy (especially the heavy profile barrel models). Expensive! I seldom see one for less than 2000 used. Unless it has a high round count and a messy krylon job of course.
Tavor/X95: A partner of mine went this route, he's very happy with his and it performs well. He paid 2400 used for the rifle with some mags and the carrying case. I find the weight to be distributed nicely and the selector lever is in the right place. I don't like the trigger. At least it's "combat proven", in case you're planning on going to war with it.
ACR: A nice rifle, very similar in concept to the xcr. Quick change barrels, if that's something necessary for you. It's also $3000 minimum for a NR, used example. For what it's worth, I think it's the best looking rifle out of the lot of them.
Modern Varminter: Damn our laws. How much are you willing to pay for a NR AR-15?