Marco is the one most like Rachel and they like each others company- they are both loud and funny. Tobias is her opposite but you can see the way he cares for her and her him. if Tobias changes to human he would be great for her because he's calm and sensible, but has a wild side of his own, but Marco could also fill his shoes too.i'll say Tobias because he can calm her down while Marco will only make her worse :D
Tobias. Definetly. Marco would likely make Rachel furious alot, which would problobly result in her morphing to elephant, and stomping on him. (JK! But still...) Rachel is the only one tying Tobias onto humanity, and if he lost Rachel, he would lose it.