Best AR for what?
If you want something to put on a smart platform and guard your decker, I'd say the Monsoon with a smart safety. Good firepower, and you won't have to reload it for a while.
Before smartsoft and pilot increases, the best weapon for drones was the -97 or -98. Now I'm less sure; still thinking it over.
if you want a smartlinked AR without an under-barrel weapon, again the monsoon. The Raiden is better, but also harder to get, and not chargen legal
If you want the best AR out there, the Alpha is the undisputed Cadillac. It's also expensive, hard to get your hands on, and forbidden. If you took the firearms group (And have longarms/shotgun but not heavy weapons/grenade-launcher) the Nissan optimum might be a better, if softer-hitting, option.
The best supressive bang for buck on a straight AR, is probably the AK-97. If you don't mind your weapon being forbidden, the AK-98 adds a grenade launcher for only Y300.
The M23 makes a good barter item, or something cheap to put in an emergency stash.
If you just need a lot of bullets to reduce that adept's defense pool, the HVAR is the way to go, despite how weak it is per bullet.
Best restricted is a tossup between AK-97 or FN HAR. If not using erratta (And not desiring aurofire,) the rainforest was king in this slot.
Of course, all this depends on the user being strong enough to control bursts. on a normal-ish human, you may be better off with the longarm skill and a sporting rifle or shotgun.