With that question you will get a million different answers. Some not so educated as others and many of them will be purely opinion based. Read the link below. It will answer most of your questions and you'll have a better idea of what you should be looking for.

http://forums.officer.com/showthread.php?81462-So-you-want-to-buy-an-AR-15-huh

 

True we all will have our own opinion of witch AR is the best I know what mine is. Years of building, changing, shooting, and I have 11 of them. All for different type of shooting. So witch is best you will have to find out for your self.

 

I've liked my Bushmasters and CMMG's.

 

Noveske, lmt, Daniel defense, colt, bcm are all top choices.
Buis, I like troy.
Optic will be determined by your goals. I run an aimpoint m4s in a bobro mount.

 

^That.

Buis = (back up iron sights) I like Troy as well but I just bought a Magpul buis and love it.