This app was mentioned in 4 comments, with an average of 1.75 upvotes
link me : Blackplayer music player, Shuttle music player , pulsar music player , N music(material) ,Retro music player
All players are ad free. All are free , some of them has in app purchase if you like it. Try blackplayer Ex , It's paid , but it has so many options.
Edit : correct link for N music player link - https://play.google.com/store/apps/details?id=com.cnj.nplayer
I don't know how simple you want your music player to be so Here's a couple that have good design.
Phonograph Phonograph probably has the most customization with colors.
Rave Music Rave is still new but can create playlists and is dead simple with gesture controls.
N Music is very similar, but without the ads