least attractive piercing is ether nose or lip piercing or any other mic place like that because it doesn't look conferable and just unpleasant to the eyes. only thing that would be less attractive than that would be piercing of private parts (down below) that's just nasty.
The other day I saw this guy who had a conch piercing with a huge barbell, probably 2 or 4 gauge, through it. It is probably the single stupidest looking piercing I have ever seen. Besides something ridiculous like that I would say septum piercings are least attractive. Most piercings can look good on certain people, but I don't think septums look good on anyone.
nipple rings can look Extremely Hot on some guys..mm
i don't really have a least favourite type of piercing since I'm really into body modification, but I guess a septum would be really hard to pull off. however of course, too many piercings crowded together is just fugly and tasteless.