For me its Zune Marketplace. I have had a Zune Pass since the Zune came out in 2006, I had looked at other online music stores and subscription accounts, and this one struck me as much better.
I can generally find all the artists I need. The usual suspects tho are not available for digital download like AC/DC, Beatles, Led Zeppelin (only purchase). The record companies are still trying to figure out how to make money with online subscription.......Eventually I think everything will be availble online.
I can download all I want for a flat fee, if I like something then I usually buy it or go ahead and buy the album or mp3 online. With the Zune Pass you do get 10 free downloads per month, so that helps keep the actual cost down below US$10 per month.
As much as I like to explore new music this is the best service fro me.