Here’s my “instant” thoughts although nothing is ever one size fits all.
I’d find WMV9 safest for quality and reach.
Flash8 (if you mean On2 VP6 for example) requires Flash8 (won’t work with earlier players) and I’m not that crazy about the quality of the earlier/other Flash codecs for video. They work if you’re building that kind of interactivity but I wouldn’t use it otherwise. VP6 is good (and slow to encode) though. I’d give it more time for people to update to Flash8 Player before using.
H.264 looks great but requires Quicktime 7. Many people, especially Windows users, have not or will not update to Quicktime 7 though. Some complain of conflicts. Some Windows users abhor Quicktime, some corporate IT environments won’t allow it or won’t allow the update.
Many people though, who use their computers for entertainment (movie trailers for example) DO have Quicktime 7 (because they’re quality junkies) so if that’s your target H.264 is good.