I don't take the word Support here literally. Support as in point of story telling, in other words pretty much any character that isn't main character or villain. Orihime is boring, Schierke is pretty good, but a little annoying at times. I haven't read D.Gray-man in ages, so can't really vote for Road either. So, my choice was between Erza and Robin. Both are lovely, and I ended up choosing Robin. I love her sense of humor.
I choose Erza Scarlet From Fairy Tail
I don't know Schierke,so I can't vote for her.Orihime?The walking dragonball with huge boobs that keeps on crying?No thanks.Erza gets a no as well,since I can't really bring myself to care for her.Robin is a great character and I love how different she is from the rest of the crew,but I feel as if she's getting a little boring lately,especially after the time-skip(hate her new design>__>).
So yeah,Road gets my vote here.I always had a weakness for her.Her first appearance left quite an impression on me already.Also loved her interactions with Allen and the fact that you can't really put her anywhere.She feels like a wildcard,even though she should be on the villain's side.