if you cant answer what is best then :
what is the easiest ?
what has the best performance ?
In my experience I would say that Starling is the most easiest and Genome2d has the best performance, although it is the most difficult to get started with.
Also I would say that at the moment (AIR 3.2 / 3.3 Beta), if you are targetting iOS, you can still gain better performance with GPU (+some optimizing) compared to Starling at least. This does not apply always, but at least in many cases. Here are some benchmark results: http://esdot.ca/site/2012/performance-comparison-starling-vs-gpu-rende r-mode
I prefer Flare3D due to its 3dsmax integration. It's so nice to make an object with Flare3d materials, export it using a single menu option, open that 3d object in flash and manipulate it in a simple way. Kudos for using the names I name objects in 3dsmax to access them in ActionScript (even groups). Very happy with their tutorials like YellowPlanet to give you a quick jumpstart in gaming.
Free with the stipulation (that I consider minor) that you will show an Alternativa3D logo and link to their site.
Flare3D is $500 but Alternative3D is a lot more sophisticated and therefore difficult to pick up easily. I was up and running in Flare3D in 1 day.
That's no longer the case (no more logo/link requirement).