Rahvin, Sammael, Lanfear, Semirhage, Asmodean...their aliases were all characters we were familiar with. Graendal is the exception. The Forsaken can't seem to resist being someone rather involved in events. Plus, it's a fictional story...where would the drama be if the reveal turns out to be someone we barely know? So I doubt it's any of those three, unless in the next book they are explored further and it turns out one of them had a hand in many of the events of the White Tower or something.

But assuming it's someone we already know fairly well, I doubt it's Laras. Verin seemed to know that she is, what was it, "many more things than just a cook" or something (can't remember the quote off-hand). If Verin was spilling all the secrets of the Dark, wouldn't she let Egwene know right off that, oh, one of the Chosen happens to be our cook? Doesn't make sense to me that she wouldn't say something.

A couple more thoughts:

-As pointed out above, she could say, on the Oath Rod, "I am not Black Ajah" and get away with it - she's not BA, she's of the freakin' Forsaken. So the whole Oath thing doesn't preclude her from being an Aes Sedai by any means.

-That said, it seems more likely still that she could be an Accepted or a Novice, or a servant of some kind. Or would her ego not allow such a demotion?

-The three names are just red herrings, setting up a hunt for Mesaana for next book...and then at the end it's probably revealed it was someone else all along or something.