Divx Pro. You don't have to worry about finding a good build or anything, and it provides superior compression. It has two disadvantages: It isn't free, and it isn't always fast (although much of the latter depends on the other software you're using). But it is still the best, and I like it the most. Xvid is usually pretty good too, if you've got a reliable build (and not one of the ones that produces artifacts that make me feel ill). I'd probably use Xvid over the standard (free) Divx.
WM9 has actually seen some work recently, if you check out the WMP10 site. They've got a couple of new specific-purpose video and audio codec, including a WMA meant for speech with some music at very low bitrates, a lossless WMA, and various WMV, like the image-panning one. It's fairly easy to work with, too. But when it comes down to it, xvid and divx both beat WMV9 codec on the full frame video front, and you can always use WMA, Ogg Vorbis, MP3, AAC, whatever alongside the Divx/xvid video stream.