There are two organizations that test vehicles for crashworthiness in the U.S.: the National Highway Traffic Safety Administration ( NHTSA), a government agency that's part of the Department of Transportation; and the Insurance Institute for Highway Safety ( IIHS), a nonprofit founded and funded by a group of car insurance companies. The way they test cars is not the same, so considering both agencies' ratings is important.
Without getting into the nitty gritty, IIHS tests are a little tougher to pass because they incorporate "offset" crash tests. Imagine hitting a wall square on – the force is spread out over the entire front of the car, deforming the "crumple zone" to help absorb all that energy. Most car crashes aren't in a lab, and few involve hitting a wall square on. So some IIHS tests involve a car crashing into an object with less of the front end of the vehicle involved – between 40 and 25 percent, meaning there's a lot less ability to absorb the impact energy.
To get the highest IIHS rating, headlight performance is also considered. Good headlights won't keep you from getting hurt in a crash, but they might prevent the crash in the first place. NHTSA does not consider headlight performance.
That doesn't make NHTSA ratings irrelevant, or IIHS ratings better. They're simply different, revealing different things about the car. The safest car is one that performs well on all the tests. Getting top ratings from both, which would be 5 stars from NHTSA and a "Top Safety Pick" rating from IIHS, is a pretty good indication that the vehicle is going to be safe in a wide variety of crashes.
You could visit NHTSA and IIHS and start making a spreadsheet of all the safety ratings of the cars you're interested in to figure all this out, or you could do something much easier: Visit Autoblog's Car Finder tool and explore the "Top Safety Rating" filter. First of all, using that filter eliminates any vehicle that doesn't get top safety ratings across the board from both crash test outfits.
You can apply a few further subfilters to check for specific safety features, like Lane Keeping Assist and Blind Spot Monitoring. And don't forget you can stack filters from other categories as well, like "Hauling Capacity", or general category filters like price, size, style, and transmission type.
We did this because we know you have better things to do that pour through ratings and make lists ... but that's exactly what we like to do, so there you go. Synergy!