Iso one with some or all of the following.
*a material that does Not wet out.
*pit zips or some other controlled pit ventilation preferably longer zips that go down some of both the arm and the torso.
*Pockets that work well with backpacks a plus, but no pockets acceptable
*ample Hood beak and good neck closures for minimal aqua ingress.
*a back vent flap is exceptable
*(not sure what it's called) Double zippered, so can be zipped open from the bottom so can be worn like a cape.

So basically like many BP jackets, but made of/w pvc, pu, or something like some solid waterproof nylon military surplus rain gear. Polyurethane coating extremely least preferred of the three

Of course somewhat lighter is better.
But am willing to take several weight penalties. But don't want a super thick heavy one.