As I tend to use my iPhone more as a music player than anything else, I've tried many, many alternatives and each time I find I come back to the native player because it's simple and effective and does the job, but obviously that's my own personal preference.
One of the very best alternatives is Mezzo
https://itunes.apple.com/gb/app/mezz...910188718?mt=8
The only problem with it is that the developer is a student who has, let's say, clearly prioritised his studies

. I haven't used it for a while so I don't know if he's actually gotten around to updating it, but when it was working on a previous version of iOS, it was fantastic - looked amazing and was very intuitive as well.
Smartplayer is another nice one, although it's a paid app at the moment
https://itunes.apple.com/us/app/smar...492333520?mt=8
Leechtunes as well, but also paid
https://itunes.apple.com/gb/app/leec...345439314?mt=8
Ecoute, paid again
https://itunes.apple.com/app/ecoute/...2653?ls=1&mt=8
(I got all these apps when they were free I assume

)