How would you pick a tattoo, if you decided you were going to get one? How would you pick something that your future self is most likely to be glad to have? A favorite piece of art? Follow Leeson's lead and get an economics-related tattoo? Names of family members are off-limits, as are answers like "get a small dot in my armpit that nobody would see."
I could argue that it is impossible to make a political, religious, or personal statement one is 100% sure will still be relevant 30 years hence. Here is the solution I teach to my kids -- never, ever make a fashion you can't remove (e.g. piercing OK, tattoos not OK). I have lived through leisure suits and grunge, wide ties and narrow, short skirts and long, tie dye and soft pastels. Think of tattooing as having a leisure suit permanently bonded to your body.
Some will say this is evading the question, so I will actually provide an answer. The only thing I might have conceivably chosen to ink my body with at 20 that I could probably still live with today at 48 would be something having to do with my undergrad college (Princeton). Call it the George Shultz rule.