iTunes is the best one because while Yahoo! offers unlimited downloads, it's only unlimited downloads of just over a million songs. With iTunes, you have 3.5 million songs plus thousands upon thousands of other items. Personally, I'd much rather pay $0.99 for each song out of a huge selection than $5 a month for unlimited downloads from a small library of crap. iTunes is going to be your best bet, but I'm not sure if any other "unlimited" services exist. My point is, even if they do exist, you're still going to be better off using iTunes.
Daniel.