Middle because i feel thats the safest spot plus my 8 year old will have a door seat and if you decide to run in the back some time it'll work best
Middle seat is absolutely the safest place but if there are other carseats in the back already obviously that's a concern you've got to deal with. There are great carseats that can fit side by side like Diono Radians though those are on the spendy side... make absolutely certain that whatever you get and wherever you install it that it's in correctly and securely. It looks easy but something like 80% of carseats are installed wrong so search around in your are for a licensed carseat safety technician and learn everything you need to know to get it in right every time.
I called my local police department to ask if it was legal to put them on the side or if it had to be in the middle lol. They told me there is no law about where u place it only required to obviously have it installed and buckled correctly.. well the reason I called an asked is because my 5 yr old step daughter had a booster which is wide and my back seat is not huge. Well she has a baby sister on her moms side an likes to mess with the baby and annoy her so I wanted to make sure I could place my daughter on the side away from her when she is at my house. They said middle is 40% safer tho. But it is a personal preference
I put my daughter in the center. It's the safest place.