Ive done Slacker and Pandora. But I think I prefer last.fm which is also available. I have the last.fm toolbar for Firefox ect so last.fm would be my choice. But this is good news and I really think we'll have a very high number of apps come 1st quarter 2010.
I'm coming from Blackberry and just got the Moto Droid. I always favored Slacker over Pandora for two reasons - sound quality, and cache. The advantage of caching a station is because if you're traveling and hit a dead spot (hey, even happens with VZW) your music continues to play. I sure hope Slacker comes out with the ability to cache the stations soon. Now, there's a 3rd thing Slacker for android has that didn't on the BB, and that is the ability to immediately start playing the last station you were playing without having to select your station each time.Update: Just played with the app on a friends G1 and it is awesome. It works just like you would expect it to only better. No need to cache on this like it did on the BlackBerry, the streaming is very smooth and the sound quality was great. The Widget doesn't really do anything but launch the app though.