The safest seat is the one that fits your budget, your child, your car, and one that you will correctly every single time. All car seats in the US pass the same crash test standards.
Britax is a wonderful car seat with all sorts of extra padding and foam and tethers RF and the company has a great track record of recalling before problems escalate. But if you cannot get it installed properly in your car, it is worthless in protecting your child. (The usually dont have problems installing, though. Just an example.)
There are certain car seats that those with experience do not recommend because we feel they are a waste of money. The Graco Comfortsport is one of them. It has a small shell and the harness straps are very low, so most kids dont make it much pass 3 in that seat before they are too tall. It also has installation problems. (If your DD is small, you could probably make it to 4 in that seat, but will need another seat to keep her harnessed longer.)
The other one not recommended are the 3-in-1's. They claim to RF, FF, and booster, but the problem with these is once again, the top harnessing slots are really low and most children will outgrow the harness by height long before they reach 40lbs. 4years and 40lbs is a bare minimum for boostering, but with these seats, parents are apt to booster even before that beause they spent the money on the seat.