What Bitt said is correct, but ISTR that there also seems to be some consensus on the audiophile boards that the alt presets and the r3mix presets are ever better that the standard ones. (Basically these settings set the other lame variables to try to get the best sound at good file size.)

I believe that the alt presets are currently "the best", you can find plenty of examples on the web, e.g. here. This information was correct around 3.92, I'm not sure what's happened since then.

John