Anyone of the popular ones would be a safe investment.
They're popular because they're acceptably decent as a sim to the majority of their users. And being popular, they've probably got their own forums and support somewhere on the internet. You can swing into those forums and get a fair idea of the value of them to some of their customers. For example, G3 has a large following and has a number of special interest forums on their website. They probably have the largest number of copies sold, but don't expect their forums to be very busy however. A lot of sim users don't have much to say about the sim. After all, they're more interested in RC models, and what's to say about a sim compared to what's to say about RC models.
Check G3's forums out..... One is exclusively for sharing user made models for the sim.....
http://www.knifeedge.com