Last I saw, Lame and Helix were testing out as the best quality mp3 encoders. Helix I think is owned by Realaudio now, and Lame is more commonly used by other programs.
As well as the encoder itself, some program on top of it will make it easier to encode a bunch at once, and add the tags. DBpoweramp converter (www.dbpoweramp.com) and foobar2000 (www.foobar2000.org) are both popular.