Hillwood

Blackarachnia started as a (MUCH better looking) edit of the Tarantulas model, but late in the series she got this badass Transmetal 2 form. Also, she was the first Transformer with clearly defined (and ENTIRELY UNECESSARY) breasts. She was a stone cold bitch in the most awesome way, more devious than perhaps any of the Predacons, so damn devious that she betrayed her entire worldview when a Maximal with giant missles came along (if you get my drift). Her unlikely love affair with Silverbolt was one of the high points of the series, a much more complex relationship than any we'd seen in Transformers or frankly most kids shows, with the relationship testing both parties' emotions and worldviews. Even in Beast Machines when she calmed down a bit she never quite lost her edge, and I think this form shows it best.