Shoutcast Network. A search engine for content is also built in to venerable WinAmp (still being updated, yes), altho Shoutcast isn’t quite what it once was IMO.
Magnatune.com. You can listen to free music directly via their site, but also via M3U-format streams, which work in various, popular players, such as FUBAR and WinAmp. What makes Magnatune pretty distinct is that it once served as an online Indie label, and has an incredible catalog of unique stuff you won’t generally hear elsewhere, across a load of genres.
Dunno about ‘best,’ but: