Which branch should I join to serve as a Reservist LPN?

You can join the Army Reserves, or Regular Army, under the ACASP (Army Civilian Acquired Skills Program) and access directly as an E5. Plus, because your license is part of your MOS requirements they will pay for the CEUs, training requirements, and license fees for the upkeep of your license. They'll also offer opportunities for advanced training and certifications, pay for them, and pay you for attending them while serving.

If you are wanting to work as an LPN in the military, you must join a branch that has them and last I knew the Air Force and Navy certainly do not. The only branch I know of that you will actually use an LPN license is the Army. The other branches will tell you they use LPNs but really all they are is medics with a larger scope of practice. Stateside I cannot assign them patients like I can assign you patients. Granted this can change as an RN I do not go around checking LPN policy every year but I would be surprised if it has. Most likely the best option for you will be the Army Reserves as they have the most medical for the Army that is not active duty.

Go to a recruiting office which has multiple service reserve recruiters and see who offers you the best deal rank wise. Those things change.