Music App for Galaxy S2

The best free music player app, by far, is TuneWiki. Especially if you like to see the lyrics while you listen to music; it has time-paced lyrics that appear on screen while your music plays (you can turn that off if you want, however it works well), and there's a bunch of other cool features too. Best thing imo is that it comes with a SongID for free, so you can hold your phone up to a radio/music player and see the name of the song that's playing.

If you're willing to pay, you definitely want to get PlayerPro. It is by far the best music player on the Android market today. It has pretty much every feature that any other music player has, one hell of a sexy UI, and social-networking integration. It costs $4.51, but there is a 5-day free trial.

There's also PowerAmp, probably the most popular music player. It's awesome, has some additional EQ options (which I think PlayerPro has as well btw) to make your audio tracks sound better and clearer. It costs $5.00, I believe, but I think there's a 15-day free trial that comes with it.