I also would have to go with Saber. Her noble phantasm would take her out. I also wish in some ways the ending was different.

Revy レヴィ, from Black Lagoon

Yeah Seras is definately stronger but shes not as sadistic as Nyu when shes Lucy. If she drank blood I think she would be able to wreck her if she really wanted. I think it'd be a good match though.

That settles it. I have to just scratch a day and watch the whole series.