Which branch of the US military should I join to be a commissioned officer working a IT position in the reserve or national guard?

A lot of it is dependent on what you would rather do with your degree. The Army actually has a cyber command to which you would be able to really apply your degree. Other branches have cyber as something that support a general mission and it is not necessarily a standalone command. I am not quite sure if the National Guard has a very robust place in cyber yet. One thing also is that the military changes literally overnight sometimes. In a couple years the structure of somebody with a stem degree could be needed in a new field. I would look at what each recruiter has to offer when you are about to commission.

Simple answer. Any branch.

If you're going to be Reserve Component, then whatever branch has an opening for the job you want within driving distance. It's all well and good to compare what's "the best" branch, or career field, or what lines up to your ideal dream job. But if your state doesn't have openings, or you have to drive four hours one way to get there, then it's not really going to be a good fit for you. People will tell you that driving long distance is no big deal and you can get reimbursed for travel expenses. But after a few years of long drives every month it will get old.

So, first thing first, talk to the units around you and see what they have available to you