What is the safest convertible car seat?

There is no single 'safest' seat, since all seats sold in the US must pass the same federal crash standard. That said, Consumer Reports rated the Graco Extend2Fit 3-in-1 as providing excellent crash protection in its tests, and babygearlab noted the Graco Turn2Me posted one of the best crash-test analysis scores in its rotating-seat group. Independent testers stress that correct installation matters more than brand, so look for a seat that fits your specific vehicle and that you can install tightly every time.