One must have folder on your iPhone should be a a folder full with music apps. Now, you have a lot of apps out there but not all of them a good. IMHO the ones below are the iPhone best apps to listen music.
Also please not that there are other “music apps” as well, but there might not be for listening music on your iPhone/ iPod Touch but to either recognize tunes (Like Shazam and SounHound ) or to make music.
Do you think there is any better music app for iPhone? Let us know in the comments