Newt, obviously.
Newt lacks the staying power to have an effect in the upcoming caucuses, and his appeal there is limited anyway. However, a weak Romney coming off of two big losses could actually lose states that were either safely with him or fairly safe (Nevada, Maine and Colorado come to mind).
Add to this the fact that the possibility of a long drawn out nomination fight (and possible late entry candidate) could give Paul the balance of power in terms of delegates.