My personal opinion:
If you have a very large library and/or need to conserve space, use m4a with a moderate bitrate of around 160 or 192 kBit. This format sound better in lower bitrates than mp3.
Mp3 files with 256 or 320 kBit are also absolutely fine.
If space is no issue at all you might choose a lossless format like Apple lossless or FLAC.