Now we're getting somewhere. The more specific parameters you give, the easier it is for us to give advice. Tons of options out there.
Thanks to everyone that answer. You help me a lot!!!
I will give more details. I want to put a scope.
How powerful of a scope, or would a non-magnifying optic (like a red-dot or holosight) work for your purposes?
What's your optics budget?
I wiil use it to target shoot, plinking and hunting.
I believe that a 16" barrel is good enough for me.
16" is great for everybody except long-range varminters and HP-match shooters, etc.
Target shooting and plinking -- the AR is ideal in any form.
Hunting: how big a critter are you planning to hunt? In most American states, you can't use the .223 cartridge for deer, but it's good for coyotes and the like. You can, however, buy extra upper receivers for your AR in other cartridges (6.5 Grendel, 6.8 SPC, .458 SOCOM) that are quite sufficient for all kinds of game.
Again, AR-15 uppers can swap in and out in a matter of seconds. Many folks have one lower receiver and multiple upper receivers.
I want to buy it in one piece.
Any reason? Again, you save a $Benjamin by buying in two halves. Assembling the two halves takes about three seconds, it's just two pushpins that are designed to come in and out every time you clean anyway.
It just saves you that $100 of silly excise tax, which gives you more money to buy a good optic.
Hope all this helps. -MV