If I set QualitySettings.antiAliasing, then it sets value on quality level that was selected with a mouse, i.e. grey line - “Simple” in the image below:
How do I set QualitySettings.antiAliasing for level that is selected as a Default value for some platform, i.e. green checkbox - “Fastest”?
I believe you need to change the quality level first as changing anti-aliasing through code changes it for the currently selected quality setting. You can change it with QualitySettings.SetQualityLevel, then set the anti-aliasing. Unfortunately I do not know how you can get hold of which level is the default level through code.
http://docs.unity3d.com/ScriptReference/QualitySettings.html
http://docs.unity3d.com/Manual/PlatformDependentCompilation.html
Check out these links. It should be everything you need for this.