If there's a point you cease cosplaying as a character and start actually becoming them, Jen from Soylent Cosplay is pretty much there with this Liara T'Soni outfit.
Well, "outfit" doesn't really do the whole thing justice, because while the outfit itself is great, it's the make-up and prosthetic work that steals the show. If you'd told me these were test shots for a very expensive Mass Effect movie, I'd believe you.
You can see more of Soylent's cosplay work at their Facebook page.
(Also, if you've never seen it, you should check out Rana McAnear, who was originally the face model for Samara and Morinth in the actual games, but who now cosplays as the characters).