mp3が出始めた頃には、

多分フリーだとこれが最高の音質です

http://ja.wikipedia.org/wiki/LAME

まだまだmp3は現役だと思いますが音質を気にする人はAACなどを使うのではないのでしょうか？

比較記事をいくつか

日本のサイトでは、少し記事が古い(午後のこ～だ配布再開前)ですが

http://www.geocities.co.jp/SiliconValley-Oakland/3372/onsitu.htm...

海外のサイトでは

http://members.tripod.com/Milaa/mp3Comparison/encoders.html

Lame is probably the best encoder when talking about quality. The encoding speed is also quite good. So, I suggest using lame if you want the best possible quality.

On another way, gogo is also a good choice. This encoder is based on the "recent" lame 3.88 and offers also a very good quality. It is also the quickest mp3 encoder (approximately 2-4 times quicker than lame), but it offers less encoding options than lame.

So, if the encoding time is not your absolute priority, use lame that offers a slightly better quality than gogo.

On the other way, if the encoding time is your priority, use gogo.

http://www.sonarnerd.net/projects/wavcomp/

• Fraunhofer performs better than lame at higher frequencies which doesn't come without a cost at lower frequencies.
• Lame is good at low bitrates and blade is good at high bitrates.
• Apple iTunes MP3 encoder is preserving even more high frequencies than blade with higher cost at low frequencies.
• Ogg would be better than lame at variable bitrates, unless it had some issues with the high end.
• WMA and RA8 are equal in performance and take the third place after mp3 and ogg.
• Only ogg and and WMA preserve high frequency content (> 16 kHz) at low bitrates.
• Apple iTunes AAC encoder performs roughly same way as WMA, but doesn't preserve highest frequencies as iTunes MP3 or WMA encoder does. This gives better performance at 6 - 16 kHz range.

すでに回答されていますが、mp3 でフリーのものであれば LAME か、同系列の gogo が良いようです。