I am by no means an expert at this. I need to learn a lot more.
I used to provide only quicktime files but realized that linux users would never be serviced properly by that.
I haven't been happy with my mpeg results so far, so I decided to provide both quicktime and realvideo.
I've avoided windows media entirely but not for any quality reasons.
For quicktime, I use Media Cleaner 5 with the Sorenson 3 codec plus QDesign for audio. Pretty much the standard there. I provide 100k and 400k options for dialup and broadband. I like high resolutions with low frame rates because it helps avoid the postage-stamp look. I believe I currently use 192x144x10fps and 320x240x15fps. I use mono audio at very low bitrates. I believe it is 22.05k, 16kbps and 44.1k, 32kbps. It's a shame to use so much precious bandwidth on the soundtrack. I need to update my quicktime settings and re-encode my videos.
For Real, I use the Premiere plugin, although Media Cleaner does those, too. I use the stock settings for dual-ISDN (90k) and "512K DSL/cable". In addition, you need to specify resolution---320x240 and 640x480 for me. I think 640x480 is too agressive, though. All my real files are single stream.
My real files are consistently better quality than my QT ones.
I've been working some with mpeg4 but nothing to get excited over yet. I recently purchased Sorenson Sqeeze, but so far the quality is surprisingly poor and the control over bitrates just isn't there. I'm very disappointed.
Did you settle on a DVD authoring tool, yet?