I've shot most of what's out there and at one point, was even getting paid to do so.
For fighting-type use 'shorter is better' if you have the right kind of ammo for a lot of stuff.
If you know you're primarily going to be looking at long wide open spaces and want the ballistic advantages, an 18" is just about right. Still shorter than the original M16, just a bit, balances well, carries well.
No need for a heavy barrel. Unless you're laying down serious suppressive fire most ARs honestly don't heat up a massive amount. If you're looking at a precision gun, even less an issue.
Look at a LaRue PredatAR, or an equivalent 18" midweight from Noveske or BCM. Free floated lightweight rail, throw some folding irons and a good scope and mount on there and you can easily hit at 500m with a halfway decent shooter. I make no claims to being an awesome shot and "I" can do that on a man sized target regularly. With a good trigger (I recommend a Geissele SSA or DM-R) a midweight barrel and good optics (Leupold VX-III or equivalent, or better, not cheap stuff) in a good mount even in 5.56mm you can make hits at ranges people generally wouldn't believe.
No reason to go beyond a 20" barrel at all, especially depending on twist (1 in 7 or 1 in 8 being my choice) and unless you like dumping 400 rounds downrange in five minutes, realistically, no need for a bull barrel. Short, stiff barrels can be highly accurate. Seen a 12" (yes, you read that right) LaRue Stealth that in the hands of the SWAT officer who carried it pretty regularly shot sub-minute out to around 200+m.
I like 16's myself, but I mostly don't do long range work. The 18's got a bit more oomph ballistically, just enough.
) ones I've had over the years, and am even leaning towards a 16" heavy that I just bought, since it isn't excessively long when a suppressor is attached, and seems to be as accurate as I can shoot anyway. 
