It depends what you wnat - some codecs are quick, others produce decent quality at low bit rates, others aim for a small file, and there are a few that produce very high quality files. The Fraunhofer codec is one of hte better ones (although there are a number of variants of this codec, so you need to investigate the various subcodecs). The BladeEnc is pretty popular (its very fast, but the quality isn't brilliant).
If your serious, get the Fraunhofer mp3 Producer Professional, which is an encoding program (and the codec). It's pretty expensive (about $400 I think), and pretty slow to encode.
- gaffa