There are a bunch of music apps, it all depends on what you are looking for. doubleTwist is a nice app if you want to sync your iTunes library with your S5 via your PC. The syncing process is slow but all of your ratings, playcounts, etc. get transferred to and from your iTunes library and phone. I used to recommend poweramp all the time but their latest app is buggy and experiences skipping during playback. The developers have yet to answer a single forum post or e-mail that I've sent them. There's also n7, Shuttle Music Player, and literally hundreds of others.
Lastly, Google makes their own music app. I have since switched to using their music app as I subscribe to their music service. It's $7.99 a month (for me, my fiance pays $9.99 a month) and I have unlimited streaming as well as unlimited downloading to my phone and tablet for offline listening. I added my iTunes library so all of my ratings and playlists transferred over. Google Music was able to match the majority of my songs and uploaded about 760 of them (out of 12000+).