Title says it all. :'D
Most Helpful Guy
Do you mean one that downloads mp3's or one that can be downloaded to play mp3's. For downloading mp3's I would either suggest uTorrent or any internet downloader app because they're pretty much the same.
As for listening to mp3's the best app I've found is the bass booster combined with myMusic.1
Most Helpful Girl
Hmm.. to download, or to play?
To download, I use this app called kuwo. But everything is in Mandarin tho, so unless you know the language, you can't use it. You can play the music, and if you like it then you can download it.
There's another app which I use call JOOX too. It's pree cool (:0