I'll start with a disclaimer: I would be a liar if I was to say the purchase of a car is an entirely rational decision for me. I have been interested in cars since my early childhood days, and obviously you get to accumulate a lot of bias over the years. Neither am I immune to the marketing machines operated by all car manufacturers.
What I can say is that during my youth my main criteria in choosing a car was performance, at least the way it tends to be measured in car magazines. However, having owned a few cars of my own I quickly learned that while some fun can be had with cars without getting killed or arrested, the main criteria that determined my happiness with a car is the amount of negative attention it attracted from me. Negative attention comes in the shape of me having to spend money on it, me having to spend my time on it, and me being unable to rely on it because it breaks down. Other than that there is not much difference between cars: they all perform quite similarly at legal speeds. If anything, the more sportier the car the less comfortable it tends to be.
Safety does matter, though, both active and passive. The car we currently own, a Honda CRV, was purchase after I crashed my previous Toyota Corolla. That Corolla was crashed because it slipped out of control while I mis-performed emergency braking; if that car was to have ABS brakes I would have probably owned it for a few more years.
I guess you can sum it all up by saying the most important criteria for buying a car is how happy I am with it.
While happiness might sound like an ambiguous criteria, that is not the case at all. A company called JD Power comes to my aid: every year, JD Power surveys tends of thousands of car owners in various countries and asks them a simple question - how happy are you with your car?
They break the question down to how happy you are with the car's design, with your dealership, with performance etc. The point, however, is that they have a survey measuring happiness, and because of the extent of their survey and the fact it's been going on for years you can get some very meaningful results out of it.
To the best of my knowledge there is no Australian JD Power survey. The closest one is the UK one, at least as far as I can tell, and you can access the 2011 version here (as you'd be able to see, it's quite detailed). Alternatively, you can check here for the press release discussing the highlights.
If you're after further illumination, you can check on the American JD Power survey. The highlights from 2011 are here, while JD Power's own website will give you a very detailed account here. Do bear in mind, though, that the American car market is rather eccentric and fairly different to the Aussie one.
To answer the original question I set out to answer in this post, I consider the JD Power survey to be the most important asset at my disposal when choosing a car. Who am I to argue with the real life experience of tens of thousands of drivers?
If you were to look at the JD Power survey results over the years, you will find that certain luxury brands routinely get the top spots (e.g., BMW, Lexus). However, you will find Honda and Toyota always occupy the top spots too, with the distinct advantage of actually being affordable and good value for money.
No, it's no wonder at all my last two car purchases have been a Toyota and a Honda. Nor should you be shocked to find my future purchases will be of a similar nature. There is a very good reason for that: my own personal experience with cars of various makes indicates the JD Powers survey provides a very good representation of the car ownership experience. Not only with the success stories, but also with failures I will never touch again (e.g., Renault).
One last word about passive car safety. We all know about crash tests, but when you compare the scores the same car gets at different tests you will often find inconsistencies.
In Australia you can put your hands on the results of real life crashes (check here for a sample). I suspect that real life crashes, under real life scenarios, tell more about a car's safety than scripted crash testing.