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

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

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.

depends what you want to do. Army has the most openings because its the biggest but they tend to be uninteresting. You'll probably get stuck in a 6 shop somewhere hating your life. You also aren't guaranteed to be working IT in the army you'll be a signal officer and will have to work with radios as much as computer networks. I don't know how other services work it but its probably similar. TBH AF and Navy will probably be much more interesting but more competitive.

17 series—cyber. My unit is always looking. Let me know if you’re interested

Air Force

Two things. If you want to be in the reserves you will need to research what is within reasonable commute of your current living situation. Regardless of what MOS and branch you want you will be limited to what falls within that radius. Once you know what your options are you will need to figure out what fits best for you. Picking a Military branch is on the individual because each branch is unique and you have to see what works for you. I did well in the Marine Corps but would guess I would have been less successful in the Air Force due to the culture of that Branch of Service. Nothing against them but it is what it is. Good Luck.

Personally I would recommend the guard over the USAR in terms of commissioning.
The guard us multiple routes you can take to commission whereas the reserves has either ROTC or Federal OCS.
Also, you can choose your branch where I believe the reserves will assign you a branch based off how you do in OCS and your top choices.
Also in the guard you can commission with a 2 year degree and have till you make 03 to get your 4 year.