My favorite website for carseat information is car-seat.org. it's a forum but you dont even have to join to ask questions. they are VERY helpful and VERY knowledgeable about different seats and even what seat might fit best in you cars model and year.
The safest carseat is the one that fits properly in your vehicle and that you can install in use correctly every time. It doesn't matter if it's a $40 Scenera or a $280 Britax.
That being said, radicaleel is right. If you go for a convertible seat (the kind that go rear facing to forward facing) you want one that rear faces the longest. Rear facing is safer than forward facing so your child should stay rear facing until the limit of the seat. So my son will be rear facing to 45lbs in my car and 40lbs in hubbys. Who knows how old he'll be then!!! Here is some information on rear facing http://www.car-safety.org/rearface.html
I have a Graco My Ride 65 (rear facing to 40lbs and forward to 65) and a Sunshine Kids Radian XTSL (rear facing to 45lbs and forward to 80). If you have smallish cars or cars pre 2002 (when LATCH started being required), the Radian will not be a good bet for you as it does not work with every seatbelt/buckle combo and has a very high back (good for tall kids) so isn't good for small cars or tall front passengers. In my opinion, there are seats with equal or better safety ratings that Britax and for less money. The Radian wasn't cheap, dont get me wrong, but I had a Britax and got rid of it because the Graco is just as easy to use and is half the price. If you lurk around on car-seat.org they will be able to point you in the direction of some test results from Britax and why a lot of the CPST's don't use them. They're a fine seat, all seats sold in he US have to meet certain criteria, some just go above and beyond differently. Some Britax, however, like the Radian, now have a rear facing tether which makes even less movement in the event of a crash and that is a very good feature.
If one of your vehicles is a truck where the back window is right in front of baby, I would say you should definitely get a seat with either an anti rebound bar, the the True Fit Premiere, or one with a rear facing tether.
if you can, find someone (search for a CPST in your area on car-seat.org) or a store in your area that has the seats you're looking into buying and try them in your car. Not all seats work in every vehicle although if you have the LATCH system it makes it much more likely.
I bought the Radian from Diapers.com because they price match and have free shipping and returns so if it didn't work I could return it for free. I got the Graco from Babies R Us when they put out a coupon. (shameless spam- if you decide to use diapers.com let me know and I'll give you my referral code so you get 15% off your first order of most things and I get $10 credit ;))