I came here expecting to be able to select "Sentinel Beam". Where is my beloved Sentinel Beam? I demand a Sentinel Beam! I love the Bungie era versions. I love the unorthodox design considering their literally meant to be dangling off flying ancient robots. Halo 3's would be my favourite if it didn't have so many blue lights everywhere. Just a little bit of red near the 'barrel' and it would have been perfect.
The light rifle is great. I especially like how in halo 5 it is a 3-shot kill if you are zoomed in. If not for the light rifle, i'd say the splinter grenades because it is great to watch someone get disintegrated.
I like that scattershot for the potential corner shots you can pull. I love being that guy that can hold down a few enemies from around a corner, allowing my teammates to flank and drop them!
Also loved the pulse grenade from Halo 4, but I think the splinter grenade might be better in terms of playability. It keeps players from entering doorways and buys you time in any number of situations.
For Halo 4 nothing surpasses the fun of tearing through everything is sight during the Campaign with a Suppressor, but in Halo 5 the Light Rifle is one of the few things I ever trade my Magnum for. That 3 hit headshot kill is a beautiful thing.