Streaming in (format X) will require that any viewers also have a player for (format X). Flash requires a Flash plug-in, WMV requires Media Player, RealVideo requires RealPlayer, Silverlight requires (whatever the Silverlight plug-in is called), etc.
What it really comes down to is:
1) Which format is supported by your server?
2) Which format can you produce?
3) Which format has the smallest bandwidth requirement?
4) Which format is used/will be used by the majority of your viewers?
Maybe you could add a quick survey to the site, or just ask them, to get an idea of what formats they want to see.