Rocket Player is my first choice. Being a smartphone and tablet newby, I am also using Google Play and the Amazon player to see if they have features I prefer. They all work great and are plenty intuitive and feature-laden.
So far, I like two things about Rocket that don't seem to be available on the other two:
First, it is possible to tell Rocket where to scan for music. I sometimes like to scan just the SD card on my tablet and ignore the music files in the tablet's memory (I have reasons....). It might be possible to do this with the other players, but it wasn't as easy to find that configuation and I haven't found it yet.
I also like the big, full screen album cover art graphics when a song is playing, which is more to my liking than the other two players. I actually have a semi-permanent installation of my tablet up in my home stereo area and the tablet prominently displays the cover art of what's playing at the time.
I have all my songs associated with album covers. It's just somethign I like, and have always been doing even back in the stone ages (at least 10 years ago) when I was using the Sony Sonic Stage music player (actually a very nice player, I liked it better than ITunes at the time) with my old proprietary Sony "Memory Stick Walkman". I still like scrolling through the "cover art" method of searching on my IPod too. Probably because I'm old and learned to select music by picking through a shelf or box of vinyl LP albums "back in the day".
This is the only area I'm having problems with the Rocket Playert - and I'm in communication with them. When any song is playing, the correct album art shows, no problem while I'm actually playing music. However, when I'm in the grid view of the "Albums" tab, scrolling through looking for a particular album, the album artwork shown is more or less random - not correct for many if not most albums, and it also changes randomly as I scroll through - when I scroll a line of the grid off the screen, and then scroll back, many of the covers shown have changed, sometimes to the appropriate one, but sometimes to a different erroneous one. Very random and variably variable
This is only on my Samsung Tab2 10.1 tablet. I don't have the problem on my Moto X phone which syncs to the exact same music files on my laptop: I have a large "playlist" on my laptop's ITunes which contains approximately 16 gigs of albums and I sync that to each device. I also have some other playlists, but again, sync'd to both devices identically.
Anyway, I'm wondring if anyone else has seen those errors in cover art?