I use the thighs and ass cheeks, and the upper arms, but the ass cheeks are where I get the least PIP. T-400 gives me pip wherever I inject it. You have to do trial and error
It is good to rotate as many sites as you can but it comes down to the individual. Quads for example......some guys hate pinning them and some love it.
Top outer portion of my ass cheek for me personally.
I'd also poke it around with my finger to find the less sensitive spot, then swab and pin it. Works for me.