What military branch and job would be the best springboard into a federal agency?

Any military service would be of benefit. Simply to put that veterans preference on your application. If your after a federal area of investigation you want to stay away from being a military police officer. Most agencies want to train you in their way of thinking instead of having to unteach other methods. Look into military intelligence. This will open up the option of CID. You will also need your bachelors degree. One option is to go into the guard or reserve and do the one weekend a month thing while the military pays for your education. You will deploy at least once during this time giving you the veterans status. Come back, complete your degree and use the contacts that you have made in your military career to spring you into your civilian one.