What's the most popular piercing to stretch?
I don't have any hard numbers, but as far as I can gather, of people with stretched piercings, the piercing that they are most likely to have stretched is earlobes. Septum piercings are also a popular choice.
On the other hand, in terms of the proportion of people that have them that choose to stretch them, I'm pretty sure that the Prince Albert piercing is a clear winner. As far as I can gather, the overwhelming majority of PA wearers stretch their piercings up at least a few sizes at some point.
I myself have both stretched earlobes and PA. I have also experimented with stretching my lorum piercing, but at the moment it's back at the 1.6mm gauge that it was pierced at. I originally stretched it much too early and as a result it never really healed properly. I'm hoping that if I leave it as it is for a while then it will heal fully and then I will be able to keep it healthy when I stretch it again.
A slightly more unusual piercing to stretch is the tongue. It's not totally unheard of or anything, but a little less common than the others. But yesterday, for no particular reason besides curiosity, I decided to have a go at stretching mine. Turns out it's pretty easy to do, but not so easy to photograph well.
So far I have only stretched up to 2.5mm (10 AWG). I don't know whether I'll go any bigger yet, but so far I am quite happy with the new size. It's still fairly similar in feel to a regular tongue bar but just a little bit more solid-feeling.
From what I gather, tongues can be quite difficult to stretch at larger gauges, but this wasn't too hard. It took a bit of work to get the bar in (a screw-in taper would probably have helped a lot but I didn't have one) but once it was in it was fine. It feels quite tight but only very slightly sore; I assume this feeling will go away over the next few days as it settles into the new size.