Re: Who makes the most accurate and lightest...
No experience with them, sorry.
Accurate and light are inversely related. The absolute most accurate setups will tend to have barrels with heavier profiles (i.e., not light). You can save weight by using light handguards/rails (e.g., Noveske NSR, Midwest Industries Gen 2, etc), but the barrel will still be hefty.
Otherwise, if you want a step down from the absolute most accurate and heaviest barrels, one of the stainless steel barrels from Noveske, BCM, White Oak, Krieger, Douglas, Rainier Ultra Match, etc, will be very good (sub-MOA with handloads and match ammo).
You could even flute/dimple them if you really want to save some weight.
I would imagine the difference in accuracy between the heaviest barrels and the slightly lighter barrels is less than .25 inch and for all but the best shooters, there is little lost by going with one of the barrels mentioned above.
Check out the semi-auto 30-round shootout threads in this subforum to see what people are putting together and how that fits with your expectations. Consistent 1/2 MOA is very difficult to find.