Best MP3 Decoder
Since what the MP3 standard really defines is a decoder, there should be no such thing as a better or a worse MP3 decoder. As long as a decoder is accurate, it'll output the expected quality.
All the most popular decoders these days output accurate enough streams. These include:
- Winamp's FhG decoder
- mpg123, probably one of the most popular decoders, also used in foobar2000, Otachan's winamp plugin, LAME and countless other MP3 players
- MPAdec, a very accurate decoder based on mpg123
- Apollo, the most mathematically accurate decoder
- MAD, another very precise GPLd MP3 decoder
- A MAD input-plugin for WinAmp is available in this HA thread
Using any of the above should be enough for all purposes. Quality differences among them are insignificant.
Older decoders could provide bad quality, due mostly to encoder implementation errors. Such bad encoders include Winamp before version 2.666 (using Playmedia's low quality AMP engine), decoders based on old XAudio, and Digideck. Also, some older decoders did not work well with VBR streams.