I'm a fan of Colt and Rock River Arms.
Are you looking for a 20" rifle, or a 14.5" (16" civilian) carbine? Light (A1), medium (NATO), or heavy barrel (A2 HB and Match)?
Or are you looking for an 18" Special Purpose Rifle (SPR) like this Mk 12?
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Most people go for a rifle with a heavy barrel (HB) or a carbine with an M4 barrel (medium carbine weight). But the rifles are much handier and just as accurate with light barrels, about 3/4 of a pound lighter. And the M4 carbine barrel has a thinned out area behind the sight so a SOCOM armorer can mount a 40mm grenade launcher. If you are not going to be mounting one of those (of course not) then the M4 barrel is just a cache.
In the service I carried all sorts of different versions over a period of 26 years and I like them all. I even competed a bit with M16A1 and M16A2 rifles and earned some leg points. But my preference is a short barreled rifle, 18" barrel with mid gas system. I don't use night vision, thermal, and lasers anymore so I don't ornament my personal rifles like Christmas trees. But some people are into that.
On the top is a 16" barreled, mid gas system rifle. Below it is a standard 20" AR15 set up in the A2 configuration.
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Here is my favorite AR. It has a very light 18" stainless barrel made by Palmetto State Armory. It is built on a Rock River Arms lower. However, it has an A1 buttstock, 5/8" shorter than an A2 version. The caliber is 6.8 SPC. It has the 6.8 version of the ACOG sight and rear back up iron sight (BUIS). It is light, handy, and quick to bring to target. It weighs 6-1/2 pounds without the ACOG and just over 7 pounds with it. It is a dream to carry. I intend to replace the A2 front sight with a gas block and folding front sight. Mounting a front sight on a free float rail is not the best option. A sight on the barrel or on a barrel mounted rail is best.
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Here is my carbine. It is a USGI SOPMOD upper. The receiver, barrel, front sight, bolt carrier group, etc. are Colt. It has the 14-1/2" military barrel but a slightly longer flash hider is permanently attached (pinned and welded) to bring barrel length to a legal 16.1". This upper was issued with the rear backup sight, the Knights Armament rails, and the front grip. The lower is all Colt. I've added an AimPoint from another rifle. This carbine, as you see it, is right at 8 pounds.
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So, what are you looking for? What are you planning to do with it?