Best place is in the pan. That gives you the actual fluid temperature that is going back into the transmission pump and the rest of the system, and that is the information you need. Shoot for 150 degrees on average. It will run higher or lower depending on ambient temp and what you are doing with the car, but if you can maintain 150 on cruise and brief city traffic you are good to go. That generally means you have capacity for the hot spots.
Since you don't want to disturb pan, the return line is the next place to install it, although that isn't going to be the best reading. Sometimes that will be cooler than the pan average and other times hotter. Pan temp is what is important.
INEEDAGN is right about output line. No real value in that reading.