Mpeg
doesn't require codex for older machines, highest quality if run at 1 min = approx 10mb at 320x240 and doesn't compress the sound quality or picture. It's compatible with all machines and is simply the BEST way to have one format that fits all.
I would suggest though, for slower connections, to have a wmv version as well. But if you only want one, DEFINITELY go with .mpeg.
EDIT: Also, don't just take it from me... ask Quiet what format he uses and why....
