you might want to take into consideration what it will look like when you're in your 70s. For a minimum of exhilaration among your future nurses I recommend some section that doesn't massively shrink or wrinkle. Back or calf, perhaps buttock.
Personally, I consider the best place for a man to be tattoed to be nowhere
(though admittedly tattoos have the advantage that if the guy is a bore in bed, you can always look at the pics)