vixen cuz not only can she attain the ability of any animal she also gains thier proprtionate abilites. eg. a rhino beetle than can lift 800 times its body weight, and is practically invulernable for insect standards(if she weighed 150 that would be 60 tons) or a tiger beetle than that has a top speed of 6 mph(in human terms that would be 300mph)
the biggest thing ive seen beast boy turn into is a t-rex and such a craeture doesnt have a lot of maneuverablitly, plus Vixen has a killer instinct and willing to act on it while BB just wants to subdue ppl and crack jokes. the only way i would give him a fighting chance is if he turns into that beast, like he did in the cartoon.