Im partial to exterminations, myself.
Edit: of the 3 exterminations I've pulled off in Halo 5, I'm most proud of the one I got in roaming king. Mostly because it was Magnum and a grenade and obviously cleared the hill.
My other 2 were both in super fiesta. while one of them came with a killtrocity, they're definitely easier with power weapons. Pretty sure one of them took literally 2 rocket shots to get 5 enemies that happened to be bunched up on plaza.