WAV files at any sample size and bitrate. It must be in stereo, however, or it will not be accepted by the stores. If you need to convert your music, the simplest way is to use iTunes.
Insert your CD or find your audio file. Select 'preferences' in the top menu, followed by 'import settings'. Change this to 'wav encoder' and the following settings. This will import your CD or convert your files to the best format. 16 bit (sample size), 44.1 kHz (sample rate), 1411 kbps (bit rate).