I gotta go with the Focus Rifle and, Needle Rifle and Grenade Launcher, but specifically, the Focus Rifle.
It has a good design and is actually a really good weapon, it's just hard to master. Not only it's useful in long range combat, but also close quarters combat. A magnificent, unique weapon. I think it should make a comeback into Halo 6. It could also be a good addition into Halo 5, but since the major content updates are over, GAME OVER.