By child car seats I assume you mean Group 2 (or i-size 100cm+) and above? Not baby or toddler car seats? Or are you including Group 1/isize equivalent? 
Anyway.
We recently used a Diono Hip high back booster in a hire car. They don't seem to make that one any more, but what I especially liked about it is that they are designed to go up to 160cm, rather than the more usual 150cm. I found in reality with my 10yo that most normal booster seats don't go anywhere near the 150cm they promise and he was unable to fit into them once he was about 140cm tall, so has been on a backless booster since then. However, the Diono seat did actually fit him, and he said that it was very comfortable. All of their current models go up to 160cm so it's a general recommendation for any current Diono HBB seat. The Cambria 2 looks like a really good deal on their website.
In the last 2 years DS1 has also used a Cybex Solution (non isofix) high back booster - was OK but outgrown sooner than I expected and would have liked. He said it was comfy. We have also had a Britax Adventure, which I bought because I was a non driver at the time and it was excellent for this purpose - cheap but sturdy, lightweight, easy to store/use. Lovely little seat as a spare, travel seat or occasional use one. He did struggle in that one aged 6 when we used it for a very long drive. His legs went numb and we had to make him a foot rest from a suitcase. Unfortunately it got damaged in a plane, so I didn't get to see how it worked long term.
I'm planning to buy soon the Joie 360 Spin, which I think is brilliant in terms of what is offered for the price. I'm in Germany so we're getting the GT version which has the side impact pop-outs and better handle placement of the i-spin version when looking at the UK models.
I would love to see a 25kg ERF seat such as the Axkid Minikid reviewed/used by a family who wouldn't necessarily have considered rear facing at that age - the only reviews you ever see of these are by enthusiasts which is all very well but never really seems a very balanced opinion.
Would be very interested to see reviewed the Britax Advansafix IV R, as it's a seat I'm considering for the future (but can't tell if it will meet future needs), and the Apramo One, which sounds like the holy grail of car seats (25kg ERF aside) - it's rear facing and swivel to 18kg, forward facing with harness to 25kg, and then a high back booster until 150cm, and all isofix and for a compelling price ticket. It doesn't seem very well known or popular so I'd be interested to see it tested.
What I'd love to see in reviews is kids of different ages/sizes (especially the top end of weight/height ranges) fitting in to see how they do, or comparisons of different seats e.g. how much space they take up in a car in comparison to each other, the max/min height of the harness, headrest, etc. They always seem to say this one has 3 positions for the headrest and this one has 11 etc, but if the 11 positions are all millimetres apart it's not much use. The same for recline functions - it always seems such a gimmick to have loads of recline positions, when really you're going to use the most upright and the most reclined position, and want to know the difference betwee those, not so much the ones in between. Also, opinions of the child themselves if they are old enough to express them :)