Autoguns.
Funny enough, Precision weapons not working well with MF.
But autoguns do. They shoot fast, they penetrate and they crit alot.
You hit heads more often, you can hit multiple at once and again, crits. All this gets boosted really well by MF, It even gets boosted the most:
You can reach 1 headshot brake point for any horde unit, including shooters, with a gun that can spit about 1000 bullets per minute. One volley into horde should easily give you max stacks.
And you don’t really need to worry about brake points on elites, since fast firing, low damage weapons are favored by DPS, which is always gets increased by MF, unlike brakepoints for prescision weapons.
Edit: I also forgot to add that MF creates pretty big gap betwen headshot and bodyshot damage for autoguns, which results in more frequent headshot kills.
Bodyshots softens the target, headshots finishes it. Easier to gain and maintain stacks as result.