I want to get a M.S. in educ. online. I can’t decide to go with tech., read.,or math. Which is more useful?

I’m currently an elementary school teacher and would like to get a M.S. degree online. I want to get a degree in an area that would benefit me in education and perhaps in other areas as well. If I ever decided to stop teaching at the elementary level and decided to teach at the college level I want a degree that would benefit me the most at both levels. I enjoy all subjects but I can only get a Masters in one. Please help me decide on the best program. Thank you in advance!