Middle seat is technically the safest if you're able to put them there but I have mine behind the passengers seat since I have a smaller baby in the middle.
I think it depends on if you have other kids too. I prefer behind the passenger seat as it’s easier to hand things back from the driver’s seat and see in the rear view mirror.
I did behind my seat in the beginning, but recently just moved it to the middle in hopes of my son being able to climb in on his own soon