I'd recommend the Britax Regent, but if you have bucket seats you may have better luck with one of the Britax seats that harness to 65 pounds (Boulevard, Decathalon or Marathon). They are on a base wich means they can install well in a narrower seat.
If you can get to a local baby store that carries these seats and try them out in your car, that would be very helpful to you.
If the Regent won't fit in the seat, I personally would recommend you get a Boulevard for the side impact wings and additional shoulder width across the top slots of the harness. My six year old still fits in a Boulevard, whereas he grew out of a Marathon by 4.5 due to the narrower shoulder area.
If your DD is narrow through the shoulders a Marathon could work just as well. I'd stay away from the Decathalon though, as it has the narrowest shoulder area of the three seats.