From the website:
"Currently, the best format for such use is FLV (Flash Video), as almost all browsers will display it natively without need of installing extra plugins. It is for example the format used in Google Video."
You can control the size and bit rate (quality) so you can create a widely compatible video, but still make it high quality.
Disclaimer: "These opinions are my own, though for a small fee they be yours too." -- Dave Haynie