Obviously, pain tolerance varies greatly from person to person, but here are some of the places that are generally painful.
The Ankle or The Top of the Foot


Look at your foot. The skin looks thin? Precisely why it really, really hurts. The layer of fat there is almost non-existent, and it leaves just nerves and skin to take the pain.
Er, You Know Where
That particular place absolutely kills for men. I shan't elaborate, but I'm sure you can imagine why.
The Eyebrows

Fancy having permanently shaped eyebrows? You'll have to suffer for it. Apart from no fat, you've got plenty of nerves and a lovely layer of muscle there.
The Chest or Rib Cage
This place again, has no fat. If you can feel your rib cage with a little bit of tummy sucking-in, chances are it's going to hurt. Either put on some weight, or ditch the idea of getting a tattoo here.
The Elbow

Haven't a clue why you would want a tattoo on your elbow, but if you fancy one, you have been forewarned
The Eyelid
I don't recommend trying this one. You could permanently damage you eye, and the paper-thin skin that is the eye lid can be damaged by the needle hurting the delicate blood vessels in there.
Knuckles
I must say, having love on one hand and hate on the other looks very, very fetching indeed. Be prepared to suffer for beauty (or the ability to make bouncers cower) however.
So there you have it. Seven of the most painful places to get a tattoo.