Ears and belly button. Anything else risks getting caught on clothing or other areas and that just ain't cricket.
Fans of FHM will remember a story where a guy who'd had a prince albert done was receiving erm...'attention' from a laydee with a tongue piercing. In the act her tongue ring got caught on his piercing and in a panic she jerked her head back!
Guys across the country are crossing their legs and making that 'Ooooooh!!!!' face right now.
Piercings can be hot...but as with most things it depends on the person whether they are hot or not.
I've had some really hot g/friends with navel and genital piercings. Oddly enough I'm not as keen on the facial piercings...e.g lips, eyebrow, nose etc