ARs are a highly versatile group, IMHO it has the largest variability of all weapon groups.
First of all there are basic jack of all trade ARs like the phaeston or the CAR. They are good fro any character, they are clearly not the best weapons but you can get decent results. Good fro either ammo priming (incediary, distruptor, cryo) or for AP/warp.
Then there are the "stagger sticks": falcon, ADAS. they give stagger and excellent ammo application. for this reason I like them best on engineers. Human engi + falcon + incediary/distruptor ammo or QFE + ADAS + cryo/distroptor ammo. Both are wicked good easily plat viable.
Then there are the burst fire headshot weapons. These are usually very good mook killers. Valkyrie is the prime example, it excells on any weapon kit with headshot or AR bonus (TSent, TSol TGI, geth weapon kits, etc). The Argus is a bit trickier because it needs a stability bonus from somewhere (mod, gear, etc) but with it it is at least as good or even better (IMO) than the Valkyrie.
Then you have the kind of "mini-macine guns": revenant, typhoon, spitfire. The revenant is a bit tricky because you need hard cover, and most players here despises it, but it is a good gun for casters (! yes) because it has low weight yet reasonably high DPS. The thyphoon and spitfire are both topnoch weapons, best used on tanky kits with ROF increase: prime examples are the flamerlees geth trooper or the destroyer, but feel free to experiment.
Then there is the mattock a "mini-harrier", you need fast fingers to get the best out of it but it is a very god gun.
Then comes three unique ARs:Harrier, Saber and PPR.
The Harrier is the harrier, the TGI+harrier combo is one the most OP kits in the game despite the ammo shortage.
The Saber is the king of headshots AR. TSent, Tsol, Geth, infiltrators, anything you wanna headshot with, easily plat viable very powerful.
The PPR is arguably the best ram up weapon. Use it with cheese or without is (i.e. incediary or warp/AP) it is very powerful, biotic charcters (for warp priming or cheesing), tanky soldiers or just the good ol TSent it is awesome on many kits.