Though I have about 3 or 4 apps that I would consider my FAVORITES, the one that I think is most important to me is SLACKER.
I have a paid subscription, and love that I can cache as many radio stations as I want to my SD card, it is like having a fresh collection of music to play every day. I use it on my PC every day at work too, and I am really excited to see the possibilities of the upcoming ON-DEMAND subscription that they will offer. At that point, if the selection of music is on par, I am not sure why I would ever need my ipod or any other music player ever again.
The app itself could certainly use some improvement, but overall it works pretty good and makes Slacker that much better.