depends what you need it for. for a melee zer0, a slag evisceration is the best.
otherwise, a binary is great for the double shot. element is your preference. i like slag because it kinda kills 2 birds with 1 stone. healing and slag weapon. most use it for the healing rather than the damage output. so take your pick.
i feel like slag gives it some added utility over just the healing --- Crazy? Crazy with low prices on Wind Brahmin. You buy one!!