I have to be careful when attending ATMOS screenings (action films). Be as far back to the back of the theater as possible. They can be a tad too loud for my sadsack ears.
The best place is in the very center, just like the best place in front of a set of stereo speakers is directly between them.
Atmos is like "stereo" for surround sound.
A properly-created song in stereo knits the speakers together so you can hear the position of each instrument arrayed in front of you, and even hear positions of the instruments move if the sound engineers so desire.
Atmos knits the all of the surround sound speakers together so that every sound in the movie has a defined position, and the positions of each sound can be made to move around the theater to follow the movement of the action on the screen.
If you are in the dead center of the theater you have the perfect balance of sounds moving around on all sides of you.