Hmm... That entirely depends on what sort of vibe she's going for. Pantsuits or skirts can work equally well, though the skirt has a more feminine flair. Heels are by no means necessary, but can be a useful tool - Pinning a grovelling underling's head down has far more impact with the tip of a heel than a pair of loafers, after all. It's important to have severe colours - Blacks, greys, though sometimes something like a dark red can work too. Nothing too flashy. Bright colours draw the eye, but they do not command a room.

Perhaps more than anything else, it's about HOW it's worn. It's important for everything to be perfect, just as it's meant to be, without a single crease nor a stitch out of place. Self-discipline is of vital importance in setting a standard for employees, and it would be impossible for them to take an evil business woman seriously if she looks sloppy.

And big shoulderpads on the jacket. Pointy. Pointing upwards a little, if at all possible. A little 80s, but it has a distinctly evil look.