Behind the passenger always. Putting baby in the middle is too dangerous if someone hits your from behind because it can cause the seat to go through the back to the front in an instant.
The safest place is where you can get the best installation. Your car Manuel should tell you
The safest place is based on your car. Take a look at the manual, and see what it says. Every car is different.
My car manual said middle was best, but I couldn’t get it right enough due to the shape of the seat. She’s been behind the passenger rear facing
I have always had mine behind the passenger side rear facing