What is the best place on the body to get a tattoo that will move the least over the years?

I am 17 almost 18 and just enlisted in the US military, and I want to get a tattoo before I leave. My biggest concern is that by the time I am 50 or so it will be all distorted. So what’s the best place on the body to get a tattoo that moves around the least? I am going to get a tattoo that is about six inches tall by three or four inches wide. I was thinking about the shoulder, upper back, or fore arm. Any advice would be appreciated.