AR is the easiest to use, after I've spent a decent amount of time experimenting with each automatic. I once used it as my secondary full time until I got weapon mastery for it
But more recently, my preference has been the storm rifle. I decided to use it mainly because it's pretty rare to come across, and it lasts surprisingly long when used with the ammo perk. This weapon definitely takes time to realize its full potential, but when used right it will leave your enemy with devastating consequences. As most experienced halo players know, plasma weapons break shields like crazy. By knowing how to navigate the map right, you can draw opponents into more confined spaces and unleash a devastating volley of blue death on them. It's works best when fired in controlled bursts to maintain accuracy and prevent the overheat effect.
The downside of it is the expected lack of range, but it also gets you a lot of assists. Teammates easily pick off the targets that you've already weakened, but this shouldn't matter if you're a team player