First, safety ratings. Then I focus on height and weight measurment/restrictions. Like how long they can stay rear facing. I also look at overall height of the carseat rear facing for fitting in my car purposes. For my daughter's I was looking at the minimum poundage because I knew she was going to be smaller.
I have been looking at a wide age range, something easy to assemble and not to bulky
There are no “safety ratings”. All car seats are safe when used correctly. Are some possibly safer than others...yeah but we don’t have a definitive way to compare.
The best seat is one that fits your child, fits your vehicle, fits your budget, and one that you use correctly 100% of the time.
www.csftl.org is a great place to research seats and compare features, etc.