Army Slots:
Tripler (Honolulu HI)
Walter Reed (D.C.)
DeWitt (D.C.)
SAMMC (San Antonio TX)
Madigan (Tacoma WA)
Navy Slots:
Portsmouth VA
Walter Reed
San Diego
Air Force Slots:
Wright-Patterson (Dayton OH)
David Grant (Sacramento CA Area)
SAMMC
Walter Reed
SAMMC and Walter Reed are the 2 largest hospitals with the most robust rad onc departments. Both hospitals have the full range of surgical oncologic subspecialties with robust medical oncology services (peds and adult) and see virtually every type of malignancy. As mentioned, Walter Reed has the Rad Onc residency and the ability to collaborate at the NIH while SAMMC has more of a very busy private practice feel. Both SAMMC and Walter Reed have multiple protocols ongoing within their departments. Tripler, San Diego, and Portsmouth are large hospitals with plenty of volume but will lack some of the rarer malignancies. David Grant and Wright Patterson are low volume centers and treat primarily bread and butter carcinomas and hematologic malignancies with a smattering of sarcomas.
In all 3 services, if you are accepted as a board certified rad-onc after residency, you'll be practicing your specialty for the early part of your career. You will be a junior officer and will function as one of the workhorses of your department (though the volume and complexity will vary as mentioned above). If you stay in long enough to become a senior officer, you will be expected to perform more administrative duties to progress in your career. These administrative duties can involve a deployment (where you will be serving as a commander and will not be functioning as a rad-onc), or serving as a commanding officer at a medical facility that may or may not have an accelerator.
Special mention was made of the Brigade Surgeon Position that is unique to the Army above. These are mostly administrative primary care positions where doctors are attached to Army Combat Brigades. These positions are generally filled by PCPs, but due to a shortage of PCPs in the Army, certain medical specialties that were considered to have excess personnel were required to provide physicians for these spots. I know of one rad-onc who was sent to one of these spots at a base that lacked an accelerator. I'm not sure if he was able to moonlight at local civilian hospitals while in this (2 year) Brigade Surgeon position. It's unlikely (but possible) that you could be tasked to one of these positions if you did a single 4 year hitch.
Talk to recruiters if you're really serious and feel them out about where you would be stationed. Make sure your recruiter puts you in touch with the specialty leader/consultant for rad-onc in that particular service (the specialty leader/consultant is a radiation oncologist who [for all intents and purposes] decides who will staff each position in that particular service). The specialty leader/consultant will be able to give you the straight dope about upcoming staffing plans. Try and get things in writing regarding duty assignments if you can (you likely won't get cut and dry orders before you sign up, but an e-mail is ok as people generally aren't going to try a blatant bait and switch).
If your commitment is 4 years or less, you'll likely get to stay at a single duty station for your entire commitment. If that duty station is Walter Reed, SAMMC, San Diego, Madigan, or Portsmouth (I don't know anybody in Tripler's oncology related specialties), I think you'll have a good experience working with (generally) good people and getting valuable experience as a new attending. From that vantage point you can get a better vista of military medicine as a whole and whether you'd want to stay more than 4 years.