Online Music Services

I just got a Sonos set up and was wondering what's a good online service to buy.  I'm looking at either Pandora, Spotify or Last.fm.  And any other suggestions.  Right now I'm leaning to Pandora.  It's only 33 per year and I'm pretty familiar with it.  Sonos will also work with other services, those are just the ones I could think of off the top of my head.

BTW, I love Sonos.  I got the Play:3 speaker.  I chose that as I was putting it in the kitchen and did not want a giant speaker system taking up the counter.  I was a little worried about how it would sound but so far I'm pretty impressed.  It puts out really good sound, even a bit bassy.  My only real gripe with the system is the price.  I'd love to get at least 2 more speakers.  I'm hoping I get lucky and some refurbs wind up on Woot.