'If' you retranscode the file, please do not share it back to the network. That depletes the quality of shared files around the network. I'm sure there are a % of files floating around on the network that have been transcoded from one lossy format to another like mp3 to mp3 or mp4 to mp3, etc.

Since most people are above using dial up connections nowadays, AIFF/WAV/FLAC/APE/Apple Lossless, etc are the best for everyone to share if they are able to and if they have the originals (not converted from mp3, etc.) After all, AIFF/WAV are 'identical' to CD with no loss of quality at all. That's why they are referred to as Lossless file formats.

Dare I say it but I feel the same about video. For some reason people still convert videos to mpeg 1 or mpeg 2 or equivalent formats or image dimensions (VCD/SVCD pixel dimensions). Mpeg 4 & H.264 are the way to go for video (though DivX is also very good, H.264 is even better.) Ideally retaining the 'original' video pixel dimensions if they would (eg: 640 x 480 pixels or large screen/HDD).