which format (mp3, ogg, aac, mpc ) gives you the highest "quality / filesize" ratio?

thank you!

This is an impossible question to answer.
You need to figure out which bitrate you want your files to be before you can begin to decide on a format.
MPC would typically need around 150kbps to 200kbps with standard profile. If that's the bitrate you can accept then I would believe MPC is the way you want to go.
If you want to go lower you should look at Ogg Vorbis.

I believe your question is impossible to answer, without knowing what kind of file size you're looking for. As an example, the experimental 'floggy' ogg encoder gets great results per byte, but (deliberately) uses so few bytes that it still sounds poor.

Cheers, Paul

If you're ever planning to use it in a portable, use Ogg Vorbis GT3 at around -q5, or Ogg Vorbis v.1 -q6 for good quality/filesize ratio, and if portables in the next month or two don't matter, or in the next year almost, go with MPC standard definately.