"Best" is always a tough one. A lot depends on your barrel, twist rate and chamber. If you want factory loaded ammo that changes the calculus quite a bit, as there are only a couple of places that sell ammo loaded out long.
I've shot Sierra 90s out of a 16" 1/7 twist WOA SDM barrel with pretty decent results at 600 yards. That being said, I think the 80gr ELDm is a better bullet and it's what I shoot in my regular 20" Service Rifle, but I have never fired them out of either of my 16" guns. I like N140 and AR-COMP with both of those bullets, but a shorter barrel might be better served by somthing like N135.
The Sierras like to shoot close to the rifling and the Hornady's like to jump at least .035 or a little more.
From a theoretical standpoint, the highest BC bullet is going to be the best in terms of wind drift even if you shoot them slow. The Sierra 95gr. HPBT and the Hornady 90gr A-Tip should work with a 6.5 twist, but I don't know anyone shooting them. I know someone that has tried the 95s and I have some 90gr. A-Tips to try but I have a 6.5 twist on my main rifles. The person that tried the 95s has a Wylde chamber and stuck with the 77gr TMKs as they seem to shoot the best for him.