tOHUS SHALT NOT STEAL
that’s a subjective question, but your top choices will be Noveske, Sabre and BCM
i personally like Noveske, but you can’t go wrong with any of the three
Best is kind of a subjective term.
But I’ll toss out some good, reputable upper suggestions in general: BCM, Noveske, and Sabre
Best is a very subjective term but I will only report from what I’ve personally observed with my Sabre and recommend Sabre middys.
BCM, Sabre, and Noveske are the only ones to consider when it comes to a mid-length. They are all making some of the highest quality barrels out there right now. I own 2 Sabre mids, a 16" and 14.5", absolutely love’em. I plan to add a BCM and Noveske very soon.
“Best” is something I am not qualified to comment on but I can say that I am very pleased with the Noveske N4 barreled upper thet Grant at G&R built for me.
Tell him what you desire and have him build it. You wont be dissappointed.
Sabre , BCM, and Noveske are a good place to start.
I personally prefer Sabre
My next upper will be a Noveske N4 without question. I’m very happy with the performance of my Daniel Defense and LaRue uppers, but after speaking with John and a few others about the N4 uppers, I really want to try one.
Since I don’t work on or build AR’s for a living, I don’t have enough exposure to know what is the “best” midlength upper. Plus, we haven’t yet defined what is meant by “best.” But, I have been shooting AR’s for a number of years, and when I decided to get my first midlength this past year, I researched quite a bit and looked at a number of brands, which included names like Noveske, Colt, Sabre, and a few others. I ended up with the Sabre, and it has not disappointed. With around 1200 rounds downrange I have had zero problems with it. Its accuracy has exceeded what I even hoped for out of a 16" middy; it can almost compete with my Rock River Predator at distances of 100 to 200 yards. I believe that a Noveske probably gives you a slightly more “high-end” rifle, which is also a great choice, but at a higher cost. For myself, I’ve been extremely happy with my Sabre at the price I paid (about $600 for an upper with a 9" Troy rail), and would get another.
BTW, if you’re interested in Sabre middy uppers, I had a really good buying experience at Pk Firearms. They make a lot of nicely built custom middy uppers, using good namebrands combined with quality aftermarket add-ons. For example, they’ll take a bunch of Sabre middy uppers, and configure them with various rails like Troy, Daniel Defense, etc., selling as a custom package. You can see some of their listing here.
Noveske
This is the one that I have. BCM-
http://www.bravocompanyusa.com/BCM-16-Mid-Length-Upper-Receiver-Group-p/bcm-urg-mid-16.htm
I am very happy with my Daniel Defense SPR 16" mid-length upper.
From an accuracy stand point the Daniel Defense SPR or Larue Tactical Stealth uppers make a lot of sense with the stainless Lothar Walther barrels.
Cameron
the others are great, but the BCM is without a doubt the best value in a middy upper on the planet. I don’t beleive the others are any better or can out shoot the BCM . as far as uppers with a rail ,you could put a rail on the BCM. 
it would be great if someone would run a test and see if in fact the claim for greater accuracy is indeed fact .
The only ones I would consider are BCM, Noveske, and Sabre. The BCM and Sabre are great if you are on a tighter budget and you want to pick your own rail system. If you don’t mind John’s proprietary rail system then his mid-lengths are definitely worth the $$. If you’re looking to do a full-on custom upper, get with Wes @ MSTN or Grant @ G&R and they’ll hook you up with an awesome Noveske N4 mid-length built to your specs. Hope this helps.
I have two Sabre and both are good.
LET’S CELEBRATE OUR LOVE FOR YOU
go to the BCM sight and click on email me when this product available ,you will be emailed and it wont be long .
STRIKE THE FAT CAT WHILE SHE IS EATING
You shouldn’t have to fit an upper to a lower.
i had to pick up one of the BCM middies last night right while they were still in stock
they do seem like a good deal, figured i might as well strike while the iron was hot. all gone now.