What is the best setting in itune for the best songs quality

This depends on your ears and how much disk space you’re willing to allocate to the songs. Encode a few at different bitrates/codecs, play them, and pick the one which sounds best.

(127126)

AIFF for quality equal to original in most cases. ALAC for quality equal to original and taking up only about 60% of the file space as AIFF. After that it depends. MP3 and AAC encoding formats are reduced quality (the higher the bitrate setting the better the quality but the larger the file) but the question is, will you notice the difference, most people can't.