From: United Kingdom
The only compressed formats which do not lose ANY quality, are lossless ones such as Monkey's Audio or FLAC.
Compression ratio is MISERABLE - even 50% would be optimistic (using a digital noise gate, to ensure that silence is absolutely digital silence, can help).
For MP3 compression, it's hard to beat LAME with alt-preset standard or alt-preset extreme
These are variable bitrate (VBR) modes, so the encoding uses as many, or as few bits are required by the momement by moment audio complexity.
A VBR averaging a certain number of Kbit/sec is invariably better than a CBR (constant bit rate) of the same size.
But a lot of software can have trouble with VBR - older versions of Easy Cd Creator would not accept it.
Musepack (MPC) may also be worth a look - in a 3-way battle (LAME MP3, OGG Vorbis, MPC) Musepack seems to score the most wins.
It's usually said that MP3pro is tuned for lower bitrates, where it does score over MP3 by making a better job of hiding what's missing
PS. MP3 bitrates - measured in Kbit/sec
32 - Voice and low-Q music
64 - Mono music, better than AM radio quality
128 - According to some, "near CD" quality, lets call it FM
(Usually "Joint stereo" coded as middle/side)
192 - CD Quality on less demanding source
256 - Often called "CD Quality"
320 - Maximimum
If I remember rightly, alt-preset standard averages close to 192, but beats it in quality (indistinguisable from original by most people)
Alt-preset extreme averages close to 256, with a quality not really bettered by CBR 320, as it has pretty much carte blanche to use 320 on the peaks of complexity, and fall back for less demanding passages.
In the LAME encoder, VBR WORKS! - in many others, VBR didn't work properly, but now the only reason to favour CBR, is if your software can't take VBR
PS. Stereo modes are:
1. Joint Stereo (where it uses correlation between left/right to reduce the data)
2. Full stereo (where it's allowed to carry/borrow bitrate between the two channels)
3. Independent channels