'Streaming' requires special software on the end of the server, as well as specific players on the user end.
To run a true 'stream' of WMA, or ASF format, you must be running Windows Media Services on the server. The source file must be encoded in whichever bit-rate the user is capable of accepting, so there is multiple versions of the same file for hi-fi or lo-fi playback.
For RealAudio, you also require a RealServer, which they have a new fancy name for their new server package that I forget at the moment. The RA files themselves can have multi-bitrate encodes in the same file, but it is still seperate encodes, for hi-fi/lo-fi playback.
For mp3, there is several hundred different streaming servers. A simple one is nullsofts ShoutCast with 'ondemand' support. The same rules apply, as there needs to be two mp3's encoded at different bit-rates for hi-fi/lo-fi playback.
Normally, this is done on the server side, when you upload as song to a host (such as NWR, or mp3.com), the server re-compresses your file at a lower bit-rate for lo-fi playback. So the server now stores two mp3 files for hi-fi/lo-fi.
For NON-streamed applications, such as Windows Media Player, RealPlayer, and a few others, the application can 'play as downloaded', as the file format is seperated into chunks that can be read seperate from the rest. This does not require anything other than a standard HTTP server.
The 'best' format depends on use, how much you got to spend on servers, or space on servers that will host you, and what kind of music and what quality you expect.
Windows Media ASF and WMA formats have several diffent codecs. The WM8 codec is fairly lossless, and fairly crisp. But you eliminate Mac and Linux users instantly, and anyone who does not have Windows Media Player version 8 or above, from being able to listen.
RealPlayer is of course becoming the least popular player out there. I for one do not have it installed, and you will hear many people whine about it's sound quality. The codecs, IMO, blow.
The mp3 format, of course, can be played by a multitude of players, and operating systems, with little limitation, so you will widen your audience. You also can get excellent quality from it, by using higher bit-rates, and better codecs (there is now hundreds of mp3 codecs).
There is other formats, of course, like Ogg Vorbis, Quick Time, etc.. But they of course require special software for playback, limitting your audience. Some have better quality than any I listed above!
So, IMO, your best best is to use the mp3 format. If you need any help picking out a good codec/encoder combo, just shout.
W.