Best is subjective. Because there are so many differences such as size, weight, straps, ease of use/install, etc.

I am 100% happy with Diono Radian R120 (now called the 3RX). Kids are 2, 4 and 6. All are in them using 5-point harness. They're not cheap but they are VERY well made (heavy too), they have a 10-year life, they're one of the narrowest seats around (can get 2 of them on the 60% 2nd row seat and there's still enough room for someone to sit in the 40% 2nd row seat comfortably or fold it down without issue) and lastly - they are versatile...

Seats rear-facing children comfortably from 5-45 pounds
Seats forward-facing children from 20-65 pounds in a 5-point harness
Converts to a booster for children from 50 pounds up to 120 pounds, and up to 57″ tall

 

We use diono radian seats and are very happy with them. My 3 year old currently uses one. For the 6 and 7 year old we changed to a lighter booster with a back. Lighter and easier to move if need be.

 

Not Pilot specific..but..In my experience with booster seats I find the ones with backs help keep the seatbelt positioned more securely and "correctly" as the child wiggles/moves around during a trip.

 

No offense to anyone on here and what they do with their children, but I would strictly follow what is set forth by the NHTSA and other entities that have spent a lot of time studying this and have compiled information. Kids can vary greatly in height and weight by a certain age but a lot of what seat is appropriate is based on skeletal and muscular development, so make sure you are following the correct recommendations.

Car Seats and Booster Seats | NHTSA

Sorry but I have some cousins that have moved children forward facing or into bigger car seats well before they were ready just because they were above the weight minimum... to me that's a fundamental failure as a parent to be so ignorant and put your child in danger.