If you're fairly new to AR15's like I am, i suggest reading up and learning about all the different things that make a good rifle. 10 different people will give you 10 different suggestions on picking out an AR.
Here is a good article to get you started. it also does a break-down of different AR15's and the writer's opinions on them.
http://forums.officer.com/forums/sho...y-an-AR-15-huh
As a new rifle shooter myself, i picked up a Bushmaster M4A3 Carbine.