You may find
this to be of some use.
this may also have some useful information.
In general...
The Colt 6920 is still the measuring stick that all others are compared to. Yes, it costs more than almost all other comers as well, but they are the closest thing to a military issue M4 that you can buy.
The Noveske N4 Carbine is most widely known for the barrel. They use a higher grade of steel, with a thicker chrome-lining.
LMT is the best deal going when you buy the upper, lower, and BCG separately (see my second link above). You can get a rifle that is a pubic-hair away from a Colt for about $400 less. Bravo Company USA also sells a complete upper that is just a little bit better than the LMT, and is a little bit cheaper to boot.
Charles Daly Defense (yes, THAT Charles Daly) is a newcomer to the market but is proving to be VERY promising. People are reporting actual street prices of around $1k and CD is hitting virtually all of the required specs.
The S&W M&P15 is still going through some growing pains IMHO brought on by shifting vendors. At first they were nothing more than rebadged Stags, then they went to assembly in house with one set of vendors, and now they've shifted vendors again for, at least, the BCG. Because of all this they are hard to pin down, but can still be a good value at around $900.
Sabre Defence tend to be a little pricey for what they are, but they are working to make all of their bolts and barrels HPT/MPI which is a good thing. In the meantime, if you want a factory complete rifle with a mid-length gas system they are a good bet, and their barrel steel is almost as good as Noveske's but with out the beefier chrome-lining.
Armalite have been coming on strong recently. They're carrier keys look to be getting staked better, they've gone back to the M4 feedramps, etc. They are, however, very happy with 4140 barrel steel and don't look to be changing. I consider them to be a bit pricey for the features offered.
CMMG is another one that can be hit or miss. If you know how to pick the right options on their site, you can get a rifle that (at least on paper) rivals a Colt or an LMT. The problem is that you have to know which options to pick, and even then it's no guarantee as they tend to shift vendors.
Stag is a pretty good value at their price point. If you're limited to spending $850, and can't cough up the extra $100 to assemble an LMT, then it's a good way to go. Just read through
this and bring it up to speed.
Bushmaster is just too damn expensive for what you get. You can get an equal rifle for less money, or get a better rifle for the same price. Either way, they don't make much sense.
Rock River Arms is an example of a rifle that's almost as good as the Bushmaster, but slightly cheaper. Problem is, the Stag is a better gun for less. RRA also tends to substitute red locktite where things should be staked, and prefers to alter the feedramps with a dremel. Neither of these indicate a good methodology at all.
Olympic and DPMs.. well.... not sure what to say here. They fill a niche, I suppose.
