Sickle, for sure. If you pace your fire properly, it's got the best ammo economy out of any primary (outside of the Blitzer, maybe). The armor pen leaves a lot to be desired, but aiming in first-person is accurate enough that you can shoot around armor or target small weak points from afar with relative ease in a lot of cases.
Unless it's a hot planet and I'm expecting a lot of engagements, in which case I'll go back to the Slugger. It feels like overkill for most enemies, but damn is it satisfying to use. I especially love the Slugger's sound design, it's got such a satisfying "kachunk"y feel to it.