[Fixen]

GIFs for small and monotonous files that nobody's going to pay attention to: Layout graphics, including buttons, line breaks, dividers, and other little aesthetics.

JPGs for big files that you want people to stare at: Photos, designs.

ZIPs because everybody uses it (except for my school computer, in which I had to ram my head against the wall whenever I had to use one).

In my opinion, AVIs and MPGs work well on most computers. Just make sure that you don't leave any loose ends that forces Windows Media Player to go looking for codecs that the Microsoft library never seems to have. That phenomenon's been an enemy of mine for a while.