Re: looking for an ar-15
Really depends on what you plan on doing with it.
If you want one for self defense/home protection, then look to the M4 carbine versions. Bushmaster, Colt, DPMS, Rock River Arms, Ruger, Daniel Defense, S&W, and Stag all have good guns. Prices will vary. I have a Colt LE 6920 M4 and I really like it.
If you are looking for more of a target/hunting rifle then I would look at something like the Remington R15 and some of the longer, heavier barreled offering from DPMS and others.
The most common caliber is obviously the .223/5.56 NATO but they are now offered in many different ones. DPMS really has a wide array of calibers to choose from, anywhere from the .223 to .260 to .308, all the way up to some .40 calibers offerings.
I am a stickler for quality so that is why I went with the Colt. Not saying it is better than the rest per se, just a reputation for quality and reliability. Do some research online and find out what to look for in a quality AR such as good bolt carriers, triggers, and chrome lined barrels.
The AR is a very versatile platform. Uppers and lowers can be swapped to build your perfect rifle and the accessories are endless. Quad rails, laser sights, tactical lights, different flash hiders, etc.
One thing to really pay attention to is what magazines you choose. Most ARs will come with a couple 20 round, steel mags but they are not optimal. You want to look for quality polymer mags that feed reliably. One way to check is to press down hard on either side of the follower. If it springs back up right away with no biding, it is a quality mag. If it tends to want to bind up and act stickier, it could cause you feed problems.
Good luck man and let us know what you decide on. They are a ton of fun.