Personally, I don't have any tatoos because I could probably never really come up with an image that I would be content to keep on my body forever.
During Basic training, a bunch of the idiots in my company got all manner of tattoos while they were on pass. Some of them were the typical "super patriotic, eagle jumping out of a burning building" type deals, and others were the crests for Military Police or whatever. Since it was middle of nowhere Missouri, most of them weren't that great.
However, I did get a chance to talk to people about some really meaningful tattoos they had, since almost none of my friends have tattoos themselves. My one buddy, VanMammeran, had his father's initials tattoo'd at the base of his neck, as he had passed away when he was 12.
Things like that can be very meaningful, and are probably of the few times I would consider getting tattoos. Doing things like that mark a closeness to a person, and are a constant reminder of them.
Personally, other than things like that, I would probably consider getting notable quotes from Metal Gear Solid, or from The Protomen, like "Even now there is hope", or the entirety of Snake's speech to Raiden at the end of Metal Gear Solid 2.
Of course, I would be afraid that the tattoo artist would make a typo >.<