I understand what you want, even though it's probably overkill. But in a way it gives some sort of "personal" insurance.
If you will be using qaac and encoding stereo 44.1 kHz. I don't think you can go higher than 320 kbps.
LC 44100Hz Stereo -- 64,72,80,96,112,128,144,160,192,224,256,288,320
The last public AAC listening test @ 96 kbps (July 2011) on this site says:
Quicktime CVBR ~> TVBR ~>Fraunhofer > CT > Nero
Assuming that order holds true throughout the bitrate range, just pick the first one. It may vary well be possible that a codec that is bad at 96 kbps is actually the best one at 320 kbps, but that would require more testing.
And assuming higher bitrate is always better quality, you'd pick the highest option. Note that is probably overkill by a lot, especially considering how bad of a listening environment a car can be because of all the external noise.
So you are probably looking at something like:
But in order to get an optimal bitrate value, you'd have to test them yourself, and I'm sure this is what you don't want to dedicate time to. Personally I use 128 kbps. Apple's "iTunes Plus" preset uses 256 kbps. And other users can suggest other values.