We recently bought the Evenflo Generations 65 Booster and we're very happy with it. It fits so tight into the car with the seat belt and anchor (we don't have the LATCH things that newer cars do). I chose this one because it's well tested, the head rest is adjustable, it has an inbuilt harness that goes up to 25.5kg and then you use it like a normal booster with the seat belt up to 45kg and it's a reasonable price.
I researched it for ages and when I finally decided what seat I wanted I found it to be the cheapest
HERE.
ETA; I also love it because it's really easy to adjust the tightness of the straps once DD is in it. DD loves the chest clip because she can do it up lol
Edited by Chickoin