Do remember that you can encode video to MULTIPLE bitrates (quality)'s. If possible -> flash might be the best choice. Otherwise I'd go with something quicktime supports (because you can have it set so that it plays while loading)
I'd say wmv. You can just easily insert a wmv plugin into a page, without the need to install streaming software in the server.
The quality is good for the filesize and the Windows Movie Maker is pretty neat, where you can easily edit and make WMVs; unlike divx/mpg (you can't stream divx right? not sure about mpg), where you have to play around and have some experience first with the encode.
i usually offer avi's (compressed with divx usually) and mov's so you can be sure everyone is happy. as much as i hate macs, i still don't leave them out of the loop. quicktime actually has some pretty killer quality.