Springfield Scout/Socom 16
springfield M1A family is a M14 clone in 308 caliber so it's good out to 600 yards with good optics.
a couple drawbacks are high pricetag an as far as semi auto calibers go 308 is pretty spendy but it's a good gun and springfield has a lifetime warrantee.
Sig Sauer 556/516
not a good gun at all, they are plagued with reliability issues and are considered by many to be the absolute worst combat style semi auto ever released.
Barrett Rec7 (very interested in this one)
I know very little about this. barrett is a very good company with a very good reputation for quality. they normally manufacture sniper rifles in 50bmg but have recently moved into the AR market. the rec7 is an AR15 with a gas piston cycling system installed, the gas system pretty much just makes it so you don't have to clean your gun as often, it is only a real necessity if you are firing thousands of rounds in a single outting. standard direct impingement cycled AR15s are able to fire several hundred rounds before they get dirty enough to start causing reliability issues.
Armalite (haven't reasearched as much)
armalite is a company, not a gun, you will have to be a bit more specific.
I generally recommend a good economy model AR15 for people who are looking to get into the "mall ninja" craze. palmetto state armory PSA15, the smith and wesson M&P15 sport and DPMS sportical are all good economy AR15s that are all in the $600 range and will teach you thin ins and outs of the design. if you decide later that you want a gas piston there is the Barrett, Stag, Rock River Arms, Adams arms, Ruger, and I believe spikes tactical all make AR15s with the gas piston. if you decide you just want a higher quality rifle for bragging rights then there is colt, spikes, rock river arms, adams arms and a number of other companies that I'm sure I've forgotten.
just for the record you chose a very wide range of guns ranging from sniper/scout to close quarters combat rifles, you might think about what kind of applications you want to use it for and then move from there, otherwise you are just hobbling yourself and making it difficult for others to try to help