Peony

Right in the pit.

I haven't tattooed an armpit in years, but it went as smooth as possible.  For some reason I thought it would be much more difficult (not that it was easy), but it is just skin after all, and if you start tripping out about the location that's certainly not going to help...
She sat great and it healed in just a few days.  Very positive outcome for a pretty unique spot to get tattooed...