It depends on what you may want to do with the degree. Do you want to teach a math degree would give you good prospects, there is a shortage of higher level math teachers at schools and colleges.
The thing to do is also examine what really interests you. I love history so am taking a Master's in that, because I'd like to learn more about the subject as well as use the degree to teach later.
Taking a degree because it offers the best prospects generally is not a good idea if you don't enjoy what you are studying.
As a subject not to study I would say an MBA it seems everyman and his dog have MBA's at the moment.