I chose Lauren. Here's my reasoning.
1. I don't think Sark is the ultimate baddie for the sheer reason that his loyalties are flexible. Therefore, he may end up allied with the good side to get what he wants or to keep himself alive.
2. The truth Irina talked about still hasn't come out so I still don't know if she's good or bad. That eliminates her.
3. Sloane is just creepy. I don't think he's got enough to be the ultimate evil.
4. Lauren, I see potential in. If the character is developed right and not just killed off at the end of the season (like most big bads), she could be a force to be reckoned with for many seasons to come.