Over the years, I've read a bunch of different articles and tried to answer that question myself. While the IIHS safety rates are a good start, it turns out there is a lot more to having a safe car than just having the best safety features. Crash testing is useful, but there are other factors that seem to be at least as important. The hard part is figuring out what those other factors are.
One of the biggest factors happens to be the size or weight of the vehicle. It makes sense that when a heavy car hits a light car, the lighter car will get moved more than the heavier car. Typical crash tests don't take into account vehicle weight. IIHS did some crash tests in 2009 to show how vehicle weight can affect crashes.
On the IIHS website, they do point out that crash test results are really only useful to compare within weight classes of cars. In general, heavier cars perform better.
This was also pointed in a 2015 study where a physician looked at injury claims and concluded that heavier cars and more expensive cars were safer
. Very large pick-up trucks and SUVs did the best. But the one finding I can't quite figure out is that the Mini Cooper Countryman did about 40% better than average. Although now that I look up the weight for the Countryman model, it isn't as tiny as I thought - it looks like it weighs 3300 to 3600 pounds. By comparison, the Honda Civic weighs only 2700 to 3000 pounds. Even the Honda Accord is lighter at 3100-3400 pounds.
I realized that the IIHS website has this great section where you can view personal injury claims data
Fortunately, the Subaru Legacy that I drive does well, particularly in the Midsize 4-door sedan category. The model with Eyesight looks to be the best overall in that category. I find it interesting that data supports what you would expect from collision avoidance. Unfortunately, Subaru seems to be the only manufacturer where they separate the vehicle with collision avoidance from those without. The areas where the Eyesight made the biggest improvement was in physical damage to other people's property and also bodily injury to others. The standard Legacy was 19% better than average regarding damage to other people's property. The Legacy with Eyesight was 31% better than average. The standard Legacy was 17% better than average regarding personal injury damage to other people. However, the Legacy with Eyesight was a whopping 49% better than average regarding personal injury to others. The Subaru Legacy definitely seems to be the safest mid size 4 door sedan - and when you add Eyesight it is quite a bit safer, especially for other drivers. Hopefully more people will be encouraged to purchase these safety features, as the benefit does seem to be more for other people rather than the driver and passenger (although there is a benefit for them too).
Interestingly, if you look at the large 4 door sedans (not luxury) - they really don't perform that well. None does as well as the Subaru Legacy.
Also interesting (to me at least), is that if you go to the small 4 door sedans, the Volkswagen Golf and GTI do very well, in spite of only weighing around 3000 lbs. The Golf-R AWD version did even better, and that may be due to the extra 300 pounds all wheel drive adds. I'm not sure why those cars did so well, given their size and weight. The Nissan Leaf and Chevy Volt did very well also - but they are heavier than you would expect due to the electric battery systems.
The mini and micro cars did horrible - except the mini cooper. It isn't that heavy to explain how well it did.
Luxury cars look to do even better than the non-luxury cars.
Pickups do extremely well - the larger and very large doing the best, but the small pickups doing much better than the average vehicle.
SUVs do extremely well - larger and luxury models doing the best, but they look much better than sedans overall regardless.
Station wagons do very well - but that only includes the Subaru Outback and the Volvo V60.
I was surprised that mini vans don't really look to do anything special. Vans the same.
One weird finding is that sports cars look to do very well - the Corvette looks to have very good ratings there. Is it because Corvette drivers are more careful? Or is it that the fiberglass body does a good job protecting people? That I can't explain.
I'm not sure if that the above information is helpful to you in any way, but I had fun looking through the data.
Based on what I read, the only choices I would do differently in the future would be to avoid some of the smaller cars, even though they have very good crash ratings and many are very reliable and cost effective options (such as the Honda Civic).
I got lucky that the Legacy with Eyesight looks to play out as very safe. Hopefully there will be a self driving version of it by the time I'm ready for a new car.