To be honest there's nothing called highest or lowest specialization. Mostly in various colleges each student is asked to choose his stream by second year of the study only. It depends on your preference only. Needless to say, there are top level managers with different specialization in different firms.. According to me it doesn't end with choosing your specialization, it's about how better you get at that major in your life/career/job.

For example, if you're the best HR, it depends on how well you source candidates for your company which you're going to work for in the future..

If you're an operations manager, it depends on how cost effectively you're managing the process.

If you're a finance guy, it depends on how efficiently manage the working capital.

Say you're a marketing guy, it depends on how much sales you're able to crack for your firm and how much you're able to promote your product..

So I'd suggest you to have an open mind towards your MBA than having a narrow focus towards the specialization.

All the best for your future!